Exercise 8A - KopyKitab...Class X Chapter 8 โ Trigonometric Identities Maths 5. (i) 2 2 1 cot 1...
Transcript of Exercise 8A - KopyKitab...Class X Chapter 8 โ Trigonometric Identities Maths 5. (i) 2 2 1 cot 1...
Class X Chapter 8 โ Trigonometric Identities Maths
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Exercise โ 8A
1. (i) 2 21 cos cos 1ec
(ii) 2 21 cot sin 1
Sol:
(i) ๐ฟ๐ป๐ = (1 โ cos2 ฮธ) ๐๐๐ ๐๐2๐
= sin2 ๐ ๐๐๐ ๐๐2๐ (โต cos2 ๐ + sin2 ๐ = 1)
= 1
๐๐๐ ๐๐2๐ ร๐๐๐ ๐๐2๐
= 1
Hence, LHS = RHS
(ii) ๐ฟ๐ป๐ = (1 + cot2 ๐) sin2 ๐
= ๐๐๐ ๐๐2๐ sin2 ๐ (โต ๐๐๐ ๐๐2 ๐ โ cot2 ๐ = 1)
= 1
sin2 ๐ร sin2 ๐
= 1
Hence, LHS = RHS
2. (i) 2 2sec 1 cot 1
(ii) 2 21 cos 1 1sec ec
(iii) 2 2 21 cos tansec
Sol:
(i) ๐ฟ๐ป๐ = (sec2 ๐ โ 1) cot2 ๐
= tan2 ๐ ร cot2 ๐ (โต ๐ ๐๐2๐ โ tan2 ๐ = 1)
= 1
cot2 ๐ร cot2 ๐
= 1
= RHS
(ii) ๐ฟ๐ป๐ = (sec2 ๐ โ 1)(๐๐๐ ๐๐2๐ โ 1)
= tan2 ๐ร cot2 ๐ (โต sec2 ๐ โ tan2 ๐ = 1๐๐๐ ๐๐๐ ๐๐2 ๐ โ cot2 ๐ = 1)
= tan2 ๐ร1
tan2 ๐
= 1
=RHS
(iii) ๐ฟ๐ป๐ = (1 โ cos2 ๐) sec2 ๐
= sin2 ๐ ร sec2 ๐ (โต sin2 ๐ + cos2 ๐ = 1)
= sin2 ๐ร1
cos2 ๐
= sin2 ๐
cos2 ๐
= tan2 ๐
= RHS
Class X Chapter 8 โ Trigonometric Identities Maths
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3. (i)
2
2
1sin 1
1 tan
(ii) 2 2
1 11
1 tan 1 cot
Sol:
(i) ๐ฟ๐ป๐ = sin2 ๐ +1
(1+tan2 ๐)
= sin2 ๐ +1
sec2 ๐ (โต sec2 ๐ โ tan2 ๐ = 1)
= sin2 ๐ + cos2 ๐
= 1
= RHS
(ii)๐ฟ๐ป๐ =1
(1+tan2 ๐)+
1
(1+cot2 ๐)
= 1
sec2 ๐ +
1
๐๐๐ ๐๐2๐
= cos2 ๐ + sin2 ๐
= 1
= RHS
4. (i) 21 cos 1 cos 1 cos 1
(ii) cos 1 cos cot 1ec cosec
Sol:
(i) ๐ฟ๐ป๐ = (1 + cos ๐) (1 โ cos ๐) (1 + cot2 ๐)
= (1 โ cos2 ๐)๐๐๐ ๐๐2๐
= sin2 ๐ ร๐๐๐ ๐๐2๐
= sin2 ๐ ร1
sin2 ๐
= 1
= RHS
(ii) ๐ฟ๐ป๐ = ๐๐๐ ๐๐๐(1 + cos ๐)(๐๐๐ ๐๐ ๐ โ cot ๐)
= (๐๐๐ ๐๐ ๐ + ๐๐๐ ๐๐ ๐ร cos ๐) (๐๐๐ ๐๐ ๐ โ cot ๐)
= (๐๐๐ ๐๐๐ +1
sin ๐ ร cos ๐) (๐๐๐ ๐๐ ๐ โ cot ๐)
= (๐๐๐ ๐๐ ๐ + cot ๐) (๐๐๐ ๐๐๐ โ cot ๐)
= ๐๐๐ ๐๐2๐ โ cot2 ๐ (โต ๐๐๐ ๐๐2๐ โ cot2 ๐ = 1)
= 1
= RHS
Class X Chapter 8 โ Trigonometric Identities Maths
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5. (i) 2
2
1cot 1
sin
(ii) 2
2
1tan 1
cos
(iii)
2
2
1cos 1
1 cot
Sol:
(i) ๐ฟ๐ป๐ = cot2 ๐ โ1
sin2 ๐
= cos2 ๐
sin2 ๐โ
1
sin2 ๐
= cos2 ๐โ1
sin2 ๐
= โsin2 ๐
sin2 ๐
= -1
=RHS
(ii) ๐ฟ๐ป๐ = tan2 ๐ โ1
cos2 ๐
= sin2 ๐
cos2 ๐โ
1
cos2 ๐
= sin2 ๐โ1
cos2 ๐
= โcos2 ๐
cos2 ๐
= -1
= RHS
(iii) ๐ฟ๐ป๐ = cos2 ๐ +1
(1+cot2 ๐)
= cos2 ๐ +1
๐๐๐ ๐๐2๐
= cos2 ๐ + sin2 ๐
=1
= RHS
6.
21 12sec
1 sin 1 sin
Sol:
๐ฟ๐ป๐ =1
(1+sin ๐)+
1
(1โsin ๐)
= (1โsin ๐)+(1+sin ๐)
(1+sin ๐) (1โsin ๐)
= 2
1โsin2 ๐
= 2
2
cos
= 2 sec2 ๐
= RHS
Class X Chapter 8 โ Trigonometric Identities Maths
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7. (i) sec 1 sin sec tan 1
(ii) sin 1 tan cos 1 cot sec cosec
Sol:
(i) ๐ฟ๐ป๐ = sec ๐(1 โ sin ๐) (sec ๐ + tan ๐)
= (sec ๐ โ sec ๐ sin ๐) (sec ๐ + tan ๐)
=(sec ๐ โ1
cos ๐ร sin ๐) (sec ๐ + tan ๐)
= (sec ๐ โ tan ๐) (sec ๐ + tan ๐)
= sec2 ๐ โ tan2 ๐
= 1
= RHS
(ii) ๐ฟ๐ป๐ = sin ๐(1 + tan ๐) + cos ๐(1 + cot ๐)
=sin ๐ + sin ๐รsin ๐
cos ๐+ cos ๐ + cos ๐ร
cos ๐
sin ๐
= cos ๐ sin2 ๐+sin3 ๐+cos2 ๐ sin ๐+cos3 ๐
cos ๐ sin ๐
= (sin3 ๐+cos3 ๐)+(cos ๐ sin2 ๐+cos2 ๐ sin ๐)
cos ๐ sin ๐
= (sin ๐+cos ๐)(sin2 ๐โsin ๐ cos ๐+cos2 ๐)+sin ๐ cos ๐(sin ๐+cos ๐)
cos ๐ sin ๐
= (sin ๐+cos ๐)(sin2 ๐+cos2 ๐โsin ๐ cos ๐+sin ๐ cos ๐)
cos ๐ sin ๐
= (sin ๐+cos ๐) (1)
cos ๐ sin ๐
=sin ๐
cos ๐ sin ๐+
cos ๐
cos ๐ sin ๐
= 1
cos ๐+
1
sin ๐
= sec ๐ + ๐๐๐ ๐๐ ๐
=RHS
8. (i)
2cot1 cos
1 cosec
ec
(ii)
2tan1
1sec
sec
Sol:
(i) ๐ฟ๐ป๐ = 1 +cot2 ๐
(1+๐๐๐ ๐๐๐)
= 1 +(๐๐๐ ๐๐2๐โ1)
(๐๐๐ ๐๐๐+1) (โต ๐๐๐ ๐๐2๐ โ cot2 ๐ = 1)
= 1 +(๐๐๐ ๐๐๐+1)(๐๐๐ ๐๐ ๐โ1)
(๐๐๐ ๐๐ ๐+1)
=1+(๐๐๐ ๐๐ ๐ โ 1)
= ๐๐๐ ๐๐ ๐
Class X Chapter 8 โ Trigonometric Identities Maths
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= RHS
(ii) ๐ฟ๐ป๐ = 1 +tan2 ๐
(1+sec ๐)
=1 +(sec2 ๐โ1)
(sec ๐+1)
= 1 +(sec ๐+1)(sec ๐โ1)
(sec ๐+1)
=1 + (sec ๐ โ 1)
=sec ๐
= RHS
9. 2
2
tan cot1 tan
cos ec
Sol:
๐ฟ๐ป๐ =(1+tan2 ๐) cot ๐
๐๐๐ ๐๐2๐
=sec2 ๐ cot ๐
๐๐๐ ๐๐2๐
=1
cos2 ๐ร
cos ๐
sin ๐1
sin2 ๐
=1
๐๐๐ ๐๐ ๐๐๐ร ๐ ๐๐2 ๐
=๐ ๐๐ ๐
๐๐๐ ๐
= ๐ก๐๐ ๐
=RHS
Hence, LHS = RHS
10.
2 2
2 2
tan cot1
1 tan 1 cot
Sol:
LHS = tan2 ๐
(1+tan2 ๐)+
cot2 ๐
(1+cot2 ๐)
= tan2 ๐
sec2 ๐+
cot2 ๐
๐๐๐ ๐๐2๐ (โต sec2 ๐ โ tan2 ๐ = 1๐๐๐ ๐๐๐ ๐๐2๐ โ cot2 ๐ = 1)
=
sin2 ๐
cos2 ๐1
cos2 ๐
+cos2 ๐
sin2 ๐1
sin2 ๐
= sin2 ๐ + cos2 ๐
= 1
= RHS
Hence, LHS = RHS
Class X Chapter 8 โ Trigonometric Identities Maths
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11.
1 cossin2cos
1 cos sinec
Sol:
LHS = sin ๐
(1+cos ๐)+
(1+cos ๐)
sin ๐
= sin2 ๐+(1+cos ๐)2
(1+cos ๐) sin ๐
= sin2 ๐+1+cos2 ๐+2 cos ๐
(1+cos ๐) sin ๐
= 1+1+2 cos ๐
(1+cos ๐) sin ๐
=2+2 cos ๐
(1+cos ๐) sin ๐
=2(1+cos ๐)
(1+cos ๐) sin ๐
=2
sin ๐
= 2cosec ๐
= RHS
Hence, L.H.S = R.H.S.
12.
tan cot
1 sec cos1 cot 1 tan
ec
Sol:
LHS = tan ๐
(1โcot ๐)+
cot ๐
(1โtan ๐)
= tan ๐
(1โcos ๐
sin ๐)
+cot ๐
(1โsin ๐
cos ๐)
= sin ๐ tan ๐
(sin ๐โcos ๐)+
cos ๐ cot ๐
(cos ๐โsin ๐)
=sin ๐ร
sin ๐
cos ๐โcos ๐ร
cos ๐
sin ๐
(sin ๐โcos ๐)
=
sin2 ๐
cos ๐โ
cos2 ๐
sin ๐
(sin ๐โcos ๐)
=sin3 ๐โcos3 ๐
cos ๐ sin ๐(sin ๐โcos ๐)
= (sin ๐โcos ๐)(sin2 ๐+sin ๐ cos ๐+cos2 ๐)
cos ๐ sin ๐(sin ๐โcos ๐)
= 1+sin ๐ cos ๐
cos ๐ sin ๐
= 1
cos ๐ sin ๐+
sin ๐ cos ๐
cos ๐ sin ๐
= 1
cos ๐ sin ๐+
sin ๐ cos ๐
cos ๐ sin ๐
= sec ๐๐๐๐ ๐๐ ๐ + 1
=1 + sec ๐๐๐๐ ๐๐ ๐
= RHS
Class X Chapter 8 โ Trigonometric Identities Maths
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13.
2 3cos sin
1 sin cos1 tan sin cos
Sol:
cos2 ๐
(1โtan ๐)+
sin3 ๐
(sin ๐โcos ๐)= (1 + sin ๐ cos ๐)
๐ฟ๐ป๐ =cos2 ๐
(1โtan ๐)+
sin3 ๐
(sin ๐โcos ๐)
= cos2 ๐
(1โsin ๐
cos ๐)
+sin3 ๐
(sin ๐โcos ๐)
= cos3 ๐
(cos ๐โsin ๐)+
sin3 ๐
(sin ๐โcos ๐)
= cos3 ๐โsin3 ๐
(cos ๐โsin ๐)
= (cos ๐โsin ๐)(cos2 ๐+cos ๐๐ ๐๐+sin2 ๐)
(cos ๐โsin ๐)
= (sin2 ๐ + cos2 ๐ + cos ๐ sin ๐)
= (1 + sin ๐ cos ๐)
= RHS
Hence, L.H.S = R.H.S.
14.
2cos sin
cos sin1 tan cos sin
Sol:
LHS = cos ๐
(1โtan ๐)โ
sin2 ๐
(cos ๐โsin ๐)
= cos ๐
(1โsin ๐
cos ๐)
โsin2 ๐
(cos ๐โsin ๐)
=cos2 ๐
(cos ๐โsin ๐)โ
sin2 ๐
(cos ๐โsin ๐)
= cos2 ๐โsin2 ๐
(cos ๐โsin ๐)
= (cos ๐+sin ๐)(cos ๐โsin ๐)
(cos ๐โsin ๐)
= (cos ๐ + sin ๐)
= RHS
Hence, LHS = RHS
15.
2 2
2 4
11 tan 1 cot
sin sin
Sol:
๐ฟ๐ป๐ = (1 + tan2 ๐) (1 + cot2 ๐)
= sec2 ๐. ๐๐๐ ๐๐2๐ (โต sec2 ๐ โ tan2 ๐ = 1 ๐๐๐ ๐๐๐ ๐๐2 โ cot2 ๐ = 1)
Class X Chapter 8 โ Trigonometric Identities Maths
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= 1
cos2 ๐.sin2 ๐
= 1
(1โsin2 ๐) sin2 ๐
= 1
sin2 ๐โsin4 ๐
=RHS
Hence, LHS = RHS
16.
2 22 2
tan cotsin cos
1 tan 1 cot
Sol:
๐ฟ๐ป๐ =tan ๐
(1+tan2 ๐ )2+
cot ๐
(1+cot2 ๐)2
= tan ๐
(sec2 ๐)2 +cot ๐
(๐๐๐ ๐๐2๐)2
= tan ๐
sec4 ๐+
cot ๐
๐๐๐ ๐๐4๐
= sin ๐
cos ๐ร cos4 ๐ +
cos ๐
sin ๐ร sin4 ๐
= sin ๐ cos3 ๐ + cos ๐ sin3 ๐
= sin ๐cos ๐(cos2 ๐ + sin2 ๐)
= sin ๐ cos ๐
= RHS
17. (i) 6 6 2 2sin cos 1 3sin cos
(ii) 2 4 2 4sin cos cos sin
(iii) 4 2 4 2cos cos cot cotec ec
Sol:
(i) ๐ฟ๐ป๐ = sin6 ๐ + cos6 ๐
= (sin2 ๐)3 + (cos2 ๐)3
= (sin2 ๐ + cos2 ๐) (sin4 ๐ โ sin2 ๐ cos2 ๐ + cos4 ๐)
= 1ร{(sin2 ๐)2 + 2 sin2 ๐ cos2 ๐ + (cos2 ๐)2 โ 3 sin2 ๐ cos2 ๐}
= (sin2 ๐ + cos2 ๐)2 โ 3 sin2 ๐ cos2 ๐
= (1)2 โ 3 sin2 ๐ cos2 ๐
= 1 โ 3 sin2 ๐ cos2 ๐
= RHS
Hence, LHS = RHS
(ii)๐ฟ๐ป๐ = sin2 ๐ + cos4 ๐
= sin2 ๐ + (cos2 ๐)2
= sin2 ๐ + (1 โ sin2 ๐)2
= sin2 ๐ + 1 โ 2 sin2 ๐ + sin4 ๐
= 1 โ sin2 ๐ + sin4 ๐
Class X Chapter 8 โ Trigonometric Identities Maths
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= cos2 ๐ + sin4 ๐
= RHS
Hence, LHS = RHS
(iii) ๐ฟ๐ป๐ = ๐๐๐ ๐๐4๐ โ ๐๐๐ ๐๐2๐
= ๐๐๐ ๐๐2๐(๐๐๐ ๐๐2๐ โ 1)
= ๐๐๐ ๐๐2๐ร cot2 ๐ (โต ๐๐๐ ๐๐2๐ โ cot2 ๐ = 1)
= (1 + cot2 ๐)ร cot2 ๐
= cot2 ๐ + cot4 ๐
= RHS
Hence, LHS = RHS
18. (i) 2
2 2
2
1 tancos sin
1 tan
(ii) 2
2
2
1 tantan
cot 1
Sol:
(i) ๐ฟ๐ป๐ =1โtan2 ๐
1+tan2 ๐
= 1โ
sin2 ๐
cos2 ๐
1+sin2 ๐
cos2 ๐
= cos2 ๐โsin2 ๐
cos2 ๐+sin2 ๐
= cos2 ๐โsin2 ๐
1
= cos2 ๐ โ sin2 ๐
= RHS
(ii) ๐ฟ๐ป๐ =1โtan2 ๐
cot2 ๐โ1
= 1โ
sin2 ๐
cos2 ๐cos2 ๐
sin2 ๐โ1
=
cos2 ๐โsin2 ๐
cos2 ๐cos2 ๐โsin2 ๐
sin2 ๐
= sin2 ๐
cos2 ๐
= tan2 ๐
= RHS
Class X Chapter 8 โ Trigonometric Identities Maths
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19. (i)
tan tan2cos
1 1ec
sec sec
(ii)
cos 1cot2
cos 1 cot
ecsec
ec
Sol:
(i) ๐ฟ๐ป๐ =tan ๐
(sec ๐โ1)+
tan ๐
(sec ๐+1)
= tan ๐ {sec ๐+1+sec ๐โ1
(sec ๐โ1)(sec ๐+1)}
= tan ๐ {2 sec ๐
(sec2 ๐โ1)}
= tan ๐ร2 sec ๐
tan2 ๐
= 2sec ๐
tan ๐
= 21
cos ๐sin ๐
cos ๐
= 21
sin ๐
= 2๐๐๐ ๐๐ ๐
= RHS
Hence, LHS = RHS
(ii) ๐ฟ๐ป๐ =cot ๐
(๐๐๐ ๐๐ ๐+1)+
(๐๐๐ ๐๐๐+1)
cot ๐
= cot2 ๐+(๐๐๐ ๐๐ ๐+1)2
(๐๐๐ ๐๐ ๐+1) cot ๐
= cot2 ๐+๐๐๐ ๐๐2๐+2๐๐๐ ๐๐ ๐+1
(๐๐๐ ๐๐ ๐+1) cot ๐
= cot2 ๐+๐๐๐ ๐๐2๐+2๐๐๐ ๐๐ ๐+๐๐๐ ๐๐2๐โcot2 ๐
(๐๐๐ ๐๐๐+1) cot ๐
= 2๐๐๐ ๐๐2๐+2๐๐๐ ๐๐ ๐
(๐๐๐ ๐๐ ๐+1) cot ๐
= 2๐๐๐ ๐๐๐(๐๐๐ ๐๐๐+1)
(๐๐๐ ๐๐๐+1) cot ๐
= 2๐๐๐ ๐๐๐
cot ๐
= 2ร1
sin ๐ร
sin ๐
cos ๐
= 2 sec ๐
= RHS
Hence, LHS = RHS
Class X Chapter 8 โ Trigonometric Identities Maths
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20. (i)
2
2
sec 1 sin
sec 1 1 cos
(ii)
2
2
sec tan cos
sec tan 1 sin
Sol:
(i)๐ฟ๐ป๐ =sec ๐โ1
sec ๐+1
=
1
cos ๐โ1
1
cos ๐+1
=
1โcos ๐
cos ๐1+cos ๐
cos ๐
= 1โcos ๐
1+cos ๐
= (1โcos ๐)(1+cos ๐)
(1+cos ๐)(1+cos ๐)
Dividing the numerator and
min 1 cosdeno ator by
= 1โcos2 ๐
(1+cos ๐)2
= sin2 ๐
(1+cos ๐)2
= RHS
(ii) ๐ฟ๐ป๐ =sec ๐โtan ๐
sec ๐+tan ๐
=
1
cos ๐โ
sin ๐
cos ๐1
cos ๐+
sin ๐
cos ๐
=
1โsin ๐
cos ๐1+sin ๐
cos ๐
= 1โsin ๐
1+sin ๐
= (1โsin ๐)(1+sin ๐)
(1+sin ๐)(1+sin ๐)
Dividing the numerator and
min 1 cosdeno ator by
= (1โsin2 ๐)
(1+sin ๐)2
= cos2 ๐
(1+sin ๐)2
= RHS
21. (i) 1
sec tan1
sin
sin
(ii)
1 coscos cot
1 cosec
(iii) 1 cos 1 cos
2cos1 cos 1 cos
ec
Class X Chapter 8 โ Trigonometric Identities Maths
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Sol:
(i) ๐ฟ๐ป๐ = โ1+sin ๐
1โsin ๐
= โ(1+sin ๐)
(1โsin ๐)ร
(1+sin ๐)
(1+sin ๐)
= โ(1+sin ๐)2
1โsin2 ๐
= โ(1+sin ๐)2
cos2 ๐
= 1+sin ๐
cos ๐
= 1
cos ๐+
sin ๐
cos ๐
= (sec ๐ + tan ๐)
= RHS
(ii) ๐ฟ๐ป๐ = โ1โcos ๐
1+cos ๐
= โ(1โcos ๐)
(1+cos ๐)ร
(1โcos ๐)
(1โcos ๐)
= โ(1โcos ๐)2
1โcos2 ๐
= โ(1โcos ๐)2
sin2 ๐
= 1โcos ๐
sin ๐
= 1
sin ๐โ
cos ๐
sin ๐
= (๐๐๐ ๐๐ ๐ โ cot ๐)
= RHS
(iii) ๐ฟ๐ป๐ = โ1+cos ๐
1โcos ๐+ โ
1โcos ๐
1+cos ๐
= โ(1+cos ๐)2
(1โcos ๐)(1+cos ๐)+ โ
(1โcos ๐)2
(1+cos ๐)(1โcos ๐)
= โ(1+cos ๐)2
(1โcos2 ๐)+ โ
(1โcos ๐)2
(1โcos2 ๐)
= โ(1+cos ๐)2
sin2 ๐+ โ
(1โcos ๐)2
sin2 ๐
= (1+cos ๐)
sin ๐+
(1โcos ๐)
sin ๐
= 1+cos ๐+1โcos ๐
sin ๐
= 2
sin ๐
= 2cos ec
= RHS
Class X Chapter 8 โ Trigonometric Identities Maths
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22. 3 3 3 3cos sin cos sin
2cos sin cos sin
Sol:
๐ฟ๐ป๐ =cos3 ๐+sin3 ๐
cos ๐+sin ๐+
cos3 ๐โsin3 ๐
cos ๐โsin ๐
= (cos ๐+sin ๐)(cos2 ๐โcos ๐ sin ๐+sin2 ๐)
(cos ๐+sin ๐)+
(cos ๐โsin ๐)(cos2 ๐+cos ๐ sin ๐+sin2 ๐)
(cos ๐โsin ๐)
= (cos2 ๐ + sin2 ๐ โ cos ๐ sin ๐) + (cos2 ๐ + sin2 ๐ + cos ๐ sin ๐)
= (1 โ cos ๐ sin ๐) + (1 + cos ๐ sin ๐)
= 2
= RHS
Hence, LHS = RHS
23.
sin sin2
cot cos cot cosec ec
Sol:
๐ฟ๐ป๐ =sin ๐
(cot ๐+๐๐๐ ๐๐ ๐)โ
sin ๐
(cot ๐โ๐๐๐ ๐๐ ๐)
= sin ๐ {(cot ๐โ๐๐๐ ๐๐ ๐)โ(cot ๐+๐๐๐ ๐๐ ๐)
(cot ๐+๐๐๐ ๐๐ ๐)(cot ๐โ๐๐๐ ๐๐ ๐)}
= sin ๐ {โ2๐๐๐ ๐๐๐
โ1) (โต ๐๐๐ ๐๐2๐ โ cot2 ๐ = 1)
= sin .2cos ec
= sin ๐ ร2ร1
sin ๐
= 2
= RHS
24. (i) 2
sin cos sin cos 2
sin cos sin cos 2sin 1
(ii) 2
sin cos sin cos 2
sin cos sin cos 1 2cos
Sol:
(i) ๐ฟ๐ป๐ =sin ๐โcos ๐
sin ๐+cos ๐+
sin ๐+cos ๐
sin ๐โcos ๐
= (sin ๐โcos ๐)2+(sin ๐+cos ๐)2
(sin ๐+๐๐๐ ๐)(sin ๐โcos ๐)
= sin2 ๐+cos2 ๐โ2 sin ๐ cos ๐+sin2 ๐+cos2 ๐+2 sin ๐ cos ๐
sin2 ๐โcos2 ๐
= 1+1
sin2 ๐โ(1โsin2 ๐) (โต sin2 ๐ + cos2 ๐ = 1)
= 2
sin2 ๐โ1+sin2 ๐
= 2
sin2 ๐โ1
Class X Chapter 8 โ Trigonometric Identities Maths
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= RHS
(ii) ๐ฟ๐ป๐ =sin ๐+cos ๐
sin ๐โcos ๐+
sin ๐โcos ๐
sin ๐+cos ๐
= (sin ๐+cos ๐)2+(sin ๐โcos ๐)2
(sin ๐โcos ๐)(sin ๐+cos ๐)
= sin2 ๐+cos2 ๐+2 sin ๐ cos ๐+sin2 ๐+cos2๐โ2 sin ๐ cos ๐
(sin2 ๐โcos2 ๐)
= 1+1
(1โcos2 ๐)โcos2 ๐ (โต sin2 ๐ + cos2 ๐ = 1)
= 2
1โ2 cos2 ๐
= RHS
25.
21 cos sincot
sin 1 cos
Sol:
๐ฟ๐ป๐ =1+cos ๐โsin2 ๐
sin ๐(1+cos ๐)
= (1+cos ๐)โ(1โcos2 ๐)
sin ๐(1+cos ๐)
= cos ๐+cos2 ๐
sin ๐(1+cos ๐)
= cos ๐(1+cos ๐)
sin ๐(1+cos ๐)
= cos ๐
sin ๐
= cot ๐
= RHS
Hence, L.H.S. = R.H.S.
26. (i) 2 2cos cot
cos cot 1 2cot 2cos cotcos cot
ecec ec
ec
(ii) 2 2tan
s tan 1 2 tan 2 tansec tan
secec sec
Sol:
(i) Here, ๐๐๐ ๐๐๐+cot ๐
๐๐๐ ๐๐๐โcot ๐
= (๐๐๐ ๐๐๐+cot ๐)(๐๐๐ ๐๐๐+cot ๐)
(๐๐๐ ๐๐๐โcot ๐)(๐๐๐ ๐๐๐+cot ๐)
= (๐๐๐ ๐๐๐+cot ๐)2
(๐๐๐ ๐๐2๐โcot2 ๐)
= (๐๐๐ ๐๐๐+cot ๐)2
1
= (๐๐๐ ๐๐๐ + cot ๐)2
Again, (๐๐๐ ๐๐๐ + cot ๐)2
= ๐๐๐ ๐๐2๐ + cot2 ๐ + 2๐๐๐ ๐๐๐ cot ๐
Class X Chapter 8 โ Trigonometric Identities Maths
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= 1 + cot2 ๐ + cot2 ๐ + 2๐๐๐ ๐๐๐ cot ๐ (โต ๐๐๐ ๐๐2๐ โ cot2 ๐ = 1)
= 1 + 2 cot2 ๐ + 2๐๐๐ ๐๐๐ cot ๐
(ii) Here, sec ๐+tan ๐
sec ๐โtan ๐
= (sec ๐+tan ๐)(sec ๐+tan ๐)
(sec ๐โtan ๐)(sec ๐+tan ๐)
= (sec ๐+tan ๐)2
sec2 ๐โtan2 ๐
= (sec ๐+tan ๐)2
1
= (sec ๐ + tan ๐)2
Again, (sec ๐ + tan ๐)2
= sec2 ๐ + tan2 ๐ + 2 sec ๐ tan ๐
= 1 + tan2 ๐ + tan2 ๐ + 2 sec ๐ tan ๐
= 1 + 2 tan2 ๐ + 2 sec ๐ tan ๐
27. (i) 1 cos sin 1 sin
1 cos sin cos
(ii) sin 1 cos 1 sin
cos 1 sin cos
Sol:
(i) LHS = 1+cos ๐+sin ๐
1+cos ๐โ๐ ๐๐๐
={(1+cos ๐)+sin ๐}{(1+cos ๐)+sin ๐}
{(1+cos ๐)โsin ๐}{(1+cos ๐)+sin ๐}
Multiplying the numerator and
denominator by (1 cos sin )
= {(1+cos ๐)+sin ๐}2
{(1+cos ๐)2โsin2 ๐}
= 1+cos2 ๐+2 cos ๐+sin2 ๐+2 sin ๐(1+cos ๐)
1+cos2 ๐+2 cos ๐โsin2 ๐
= 2+2 cos ๐+2 sin ๐(1+cos ๐)
1+cos2 ๐+2 cos ๐โ(1โcos2 ๐)
= 2(1+cos ๐)+2 sin ๐(1+cos ๐)
2 cos2 ๐+2 cos ๐
= 2(1+cos ๐)(1+sin ๐)
2 cos ๐(1+cos ๐)
= 1+sin ๐
cos ๐
= RHS
(ii)๐ฟ๐ป๐ =sin ๐+1 cos ๐
cos ๐โ1+sin ๐
= (sin ๐+1โcos ๐)(sin ๐+cos ๐+1)
(cos ๐โ1+sin ๐)(sin ๐+cos ๐+1)
Multiplying the numerator and
denominator by (1 cos sin )
= (sin ๐+1)2โcos2 ๐
(sin ๐+cos ๐)2โ12
= sin2 ๐+1+2 sin ๐โcos2 ๐
sin2 ๐+cos2 ๐+2 sin ๐ cos ๐โ1
Class X Chapter 8 โ Trigonometric Identities Maths
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= sin2 ๐+sin2 ๐+cos2 ๐+2 sin ๐โcos2 ๐
2 sin ๐ cos ๐
= 2 sin2 ๐+2๐ ๐๐ ๐
2 sin ๐ cos ๐
= 2 sin ๐(1+sin ๐)
2 sin ๐ cos ๐
= 1+sin ๐
cos ๐
= RHS
28.
sin cos1
sec tan 1 cos cot 1ec
Sol:
๐ฟ๐ป๐ =sin ๐
(sec ๐+tan ๐โ1)+
cos ๐
(๐๐๐ ๐๐ ๐+cot ๐โ1)
= sin ๐ cos ๐
1+sin ๐โcos ๐+
cos ๐ sin ๐
1+cos ๐โsin ๐
= sin ๐ cos ๐ [1
1+(sin ๐โcos ๐)+
1
1โ(sin ๐โcos ๐)]
= sin ๐ cos ๐ [1โ(sin ๐โcos ๐)+1+(sin ๐โcos ๐)
{1+(sin ๐โcos ๐)}{1โ(sin ๐โcos ๐)}]
= sin ๐ cos ๐[1โsin ๐+cos ๐+1+sin ๐โcos ๐
1โ(sin ๐โcos ๐)2
= 2 sin ๐๐๐๐ ๐
1โ(sin2 ๐+cos2 ๐โ2 sin ๐ cos ๐)
= 2 sin ๐ cos ๐
2 sin ๐ cos ๐
= 1
= RHS
Hence, LHS = RHS
29. 2 2 2
sin cos sin cos 2 2
sin cos sin cos sin cos 2sin 1
Sol:
We have sin ๐+cos ๐
sin ๐โcos ๐+
sin ๐โcos ๐
sin ๐+cos ๐
= (sin ๐+cos ๐)2+(sin ๐โcos ๐)2
(sin ๐โcos ๐)(sin ๐+cos ๐)
= sin2 ๐+cos2 ๐+2๐ ๐๐๐ cos ๐+sin2 ๐+cos2 ๐โ2 sin ๐ cos ๐
sin2 ๐โcos2 ๐
= 1+1
sin2 ๐โcos2 ๐
= 2
sin2 ๐โcos2 ๐
Again, 2
sin2 ๐โcos2 ๐
= 2
sin2 ๐โ(1โsin2 ๐)
= 2
2 sin2 ๐โ1
Class X Chapter 8 โ Trigonometric Identities Maths
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30. cos cos sin sec
cos seccos sin
ecec
Sol:
๐ฟ๐ป๐ =cos ๐๐๐๐ ๐๐๐โsin ๐๐ ๐๐๐
cos ๐+sin ๐
=
cos ๐
sin ๐โ
sin ๐
cos ๐
cos ๐+sin ๐
= cos2 ๐โsin2 ๐
cos ๐ sin ๐(cos ๐+sin ๐)
= (cos ๐+sin ๐)(cos ๐โsin ๐)
cos ๐ sin ๐(cos ๐+sin ๐)
= (cos ๐โsin ๐)
cos ๐ sin ๐
= 1
sin ๐โ
1
cos ๐
= ๐๐๐ ๐๐ ๐ โ sec ๐
= RHS
Hence, LHS = RHS
31. 2 2
cos1 tan cot sin cos
cos
sec ec
ec sec
Sol:
๐ฟ๐ป๐ = (1 + tan ๐ + cot ๐)(sin ๐ โ cos ๐)
= sin ๐ + ๐ก๐๐๐ sin ๐ + cot ๐ sin ๐ โ cos ๐ โ tan ๐ cos ๐ โ cot ๐ cos ๐
= sin ๐ + tan ๐ sin ๐ +cos ๐
sin ๐ร sin ๐ โ cos ๐ โ
sin ๐
cos ๐ร cos ๐ โ cot ๐ cos ๐
= sin ๐ + tan ๐ sin ๐ + cos ๐ โ cos ๐ โ sin ๐ โ cot ๐ cos ๐
= tan ๐ sin ๐ โ cot ๐ cos ๐
= sin ๐
cos ๐ร
1
๐๐๐ ๐๐๐โ
cos ๐
sin ๐ร
1
sec ๐
= 1
๐๐๐ ๐๐๐ร
1
๐๐๐ ๐๐๐ร sec ๐ โ
1
sec ๐ร
1
sec ๐ร๐๐๐ ๐๐๐
= sec ๐
๐๐๐ ๐๐2๐โ
๐๐๐ ๐๐๐
sec2 ๐
= RHS
Hence, LHS = RHS
32.
2 2cot 1 sec sin 10
1 sin 1 sec
sec
Sol:
๐ฟ๐ป๐ =cot2 ๐(sec ๐โ1)
(1+sin ๐)+
sec2 ๐(sin ๐โ1)
(1+sec ๐)
=
cos2 ๐
sin2 ๐(
1
cos ๐โ1)
(1+sin ๐)+
1
cos2 ๐(sin ๐โ1)
(1+1
cos ๐)
Class X Chapter 8 โ Trigonometric Identities Maths
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=
cos2 ๐
sin2 ๐(
1โcos ๐
cos ๐)
(1+sin ๐)+
(sin ๐โ1)
cos2 ๐
(cos ๐+1
cos ๐)
= cos2 ๐(1โcos ๐)
sin2 ๐ cos ๐(1+sin ๐)+
(sin ๐โ1)cos ๐
(cos ๐+1) cos2 ๐
= cos ๐(1โcos ๐)
(1โcos2 ๐)(1+sin ๐)+
(sin ๐โ1)cos ๐
(cos ๐+1)(1โsin2 ๐)
= cos ๐(1โcos ๐)
(1โcos ๐)(1+cos ๐)(1+sin ๐)+
โ(1 sin ๐) cos ๐
(cos ๐+1)(1โsin ๐)(1+sin ๐)
= cos ๐
(1+cos ๐)(1+sin ๐)โ
cos ๐
(cos ๐+1)(1+sin ๐)
= ๐
= RHS
33.
2 2
2 2
2 22 2 2 2
1 1 1 sin cossin cos
2 sin coscos sinsec cosec
Sol:
๐ฟ๐ป๐ = {1
sec2 ๐โcos2 ๐+
1
๐๐๐ ๐๐2๐โsin2 ๐}(sin2 ๐ cos2 ๐)
= {cos2 ๐
1โcos4 ๐+
sin2 ๐
1โsin4 ๐}(sin2 ๐ cos2 ๐)
= {cos2 ๐
(1โcos2 ๐)(1+cos2 ๐)+
sin2 ๐
(1โsin2 ๐)(1+sin2 ๐)} (sin2 ๐ cos2 ๐)
= [cot2 ๐
1+cos2 ๐+
tan2 ๐
1+sin2 ๐] sin2 ๐ cos2 ๐
= cos4 ๐
1+cos2 ๐+
sin4 ๐
1+sin2 ๐
= (cos2 ๐ )
2
1+cos2 ๐+
(sin2 ๐)2
1+sin2 ๐
= (1โsin2 ๐)
1+cos2 ๐+
(1โcos2 ๐)2
1+sin2 ๐
= (1โsin2 ๐)2(1+sin2)+(1โcos2 ๐)2(1+cos2 ๐)
(1+sin2 ๐)(1+cos2 ๐)
= cos4 ๐(1+sin2 ๐)+sin4 ๐(1+cos2 ๐)
1+sin2 ๐+cos2 ๐+sin2 ๐ cos2 ๐
= cos4 ๐ cos4 ๐ sin2 ๐+sin4 ๐+sin4 ๐ cos2 ๐
1+1 sin2 ๐ cos2 ๐
= cos4 ๐+sin4 ๐+sin2 ๐ cos2 ๐(sin2 ๐+cos2 ๐)
2+sin2 ๐ cos2 ๐
= (cos2 ๐)2+(sin2 ๐)2+sin2 ๐ cos2 ๐(1)
2+sin2 ๐ cos2 ๐
= (cos2 ๐+sin2 ๐)2โ2 sin2 ๐ cos2 ๐+sin2 ๐ cos2 ๐(1)
2+sin2 ๐ cos2 ๐
= 12+cos2 ๐ sin2 ๐โ2 cos2 ๐ sin2 ๐
2+sin2 ๐ cos2 ๐
= 1โcos2 ๐ sin2 ๐
2+sin2 ๐ cos2 ๐
= RHS
Class X Chapter 8 โ Trigonometric Identities Maths
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34.
sin sin cos cos0
cos cos sin sin
A B A B
A B A B
Sol:
๐ฟ๐ป๐ =(๐ ๐๐๐ดโ๐ ๐๐๐ต)
(๐๐๐ ๐ด+๐๐๐ ๐ต)+
(๐๐๐ ๐ดโ๐๐๐ ๐ต
(๐ ๐๐๐ด+๐๐๐๐ต)
= (๐ ๐๐๐ดโ๐ ๐๐๐ต)(๐ ๐๐๐ด+๐ ๐๐๐ต)+(๐๐๐ ๐ดโ๐๐๐ ๐ต)(๐๐๐ ๐ดโ๐๐๐ ๐ต)
(๐๐๐ ๐ด+๐๐๐ ๐ต)(๐ ๐๐๐ด+๐ ๐๐๐ต)
= sin2 ๐ดโsin2 ๐ต+cos2 ๐ดโcos2 ๐ต
(๐๐๐ ๐ด+๐๐๐ ๐ต)(๐ ๐๐๐ด+๐ ๐๐๐ต)
= 0
(๐๐๐ ๐ด+๐๐๐ ๐ต)(๐ ๐๐๐ด+๐ ๐๐๐ต)
= 0
= RHS
35. tan tan
tan tancot cot
A BA B
A B
Sol:
๐ฟ๐ป๐ =๐ก๐๐๐ด+๐ก๐๐๐ต
๐๐๐ก๐ด+๐๐๐ก๐ต
= ๐ก๐๐๐ด+๐ก๐๐๐ต
1
๐ก๐๐๐ด+
1
๐ก๐๐๐ต
= ๐ก๐๐๐ด+๐ก๐๐๐ต๐ก๐๐๐ด+๐ก๐๐๐ต
๐ก๐๐๐ด ๐ก๐๐๐ต
= ๐ก๐๐๐ด ๐ก๐๐๐ต(๐ก๐๐๐ด+๐ก๐๐๐ต)
(tan ๐ด+tan ๐ต)
= ๐ก๐๐๐ด ๐ก๐๐๐ต
= RHS
Hence, LHS = RHS
36. Show that none of the following is an identity:
(i) 2cos cos 1
(ii) 2sin sin 2
(iii) 2 2sin costan
Sol:
(๐) cos2 ๐ + cos ๐ = 1
๐ฟ๐ป๐ = cos2 ๐ + cos ๐
=1 โ sin2 ๐ + cos ๐
= 1 โ (sin2 ๐ โ cos ๐)
Since LHS โ RHS, this not an identity.
(ii) sin2 ๐ + sin ๐ = 1
๐ฟ๐ป๐ = sin2 ๐ + sin ๐
= 1 โ cos2 ๐ + sin ๐
Class X Chapter 8 โ Trigonometric Identities Maths
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= 1 โ (cos2 ๐ โ sin ๐)
Since LHS โ RHS, this is not an identity.
(iii) tan2 ๐ + sin ๐ = cos2 ๐
๐ฟ๐ป๐ = tan2 ๐ + sin ๐
= sin2 ๐
cos2 ๐+ sin ๐
= 1โcos2 ๐
cos2 ๐+ sin ๐
= sec2 ๐ โ 1 + sin ๐
Since LHS โ RHS, this is not an identity.
37. Prove that 3 3sin 2sin 2cos cos tan
Sol:
๐ ๐ป๐ = (2 cos3 ๐ โ cos ๐) tan ๐
= (2 cos2 ๐ โ 1)cos ๐รsin ๐
cos ๐
= [2(1 โ sin2 ๐) โ 1]sin ๐
= (2 โ 2 sin2 ๐ โ 1)sin ๐
= (1 โ 2 sin2 ๐) sin ๐
= (sin ๐ โ 2 sin3 ๐)
= LHS
Exercise โ 8B
1. If cos sina b m and sin cos ,a b n prove that, 2 2 2 2m n a b
Sol:
We have ๐2 + ๐2 = [(๐ cos ๐ + ๐ sin ๐)2 + (๐ sin ๐ โ ๐ cos ๐)2]
= (๐2 cos2 ๐ + ๐2 sin2 ๐ + 2๐๐ cos ๐ sin ๐)
+ (๐2 sin2 ๐ + ๐2 cos2 ๐ โ 2๐๐๐๐๐ ๐ sin ๐)
= ๐2 cos2 ๐ + ๐2 sin2 ๐ + ๐2 sin2 ๐ + ๐2 cos2 ๐
= (๐2 cos2 ๐ + ๐2 sin2 ๐) + (๐2 cos2 ๐ + ๐2 sin2 ๐)
= ๐2(cos2 ๐ + sin2 ๐) + ๐2(cos2 ๐ + sin2 ๐)
= ๐2 + ๐2 [โต sin2 + cos2 = 1]
Hence,๐2 + ๐2 = ๐2 + ๐2
2. If tanx a sec b and tan ,y a b sec prove that 2 2 2 2 .x y a b
Sol:
We have ๐ฅ2 โ ๐ฆ2 = [(asec ๐ + ๐๐ก๐๐ ๐)2 โ (atan ๐ + ๐๐ ๐๐ ๐)2]
= (๐2 sec2 ๐ + ๐2 tan2 ๐ + 2๐๐๐ ๐๐๐ tan ๐)
Class X Chapter 8 โ Trigonometric Identities Maths
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-(๐2 tan2 ๐ + ๐2 sec2 ๐ + 2๐๐๐ก๐๐๐ sec ๐)
= ๐2 sec2 ๐ + ๐2 tan2 ๐ โ ๐2 tan2 ๐ โ ๐2 sec2 ๐
= (๐2 sec2 ๐ โ ๐2 tan2 ๐) โ (๐2 sec2 ๐ โ ๐2 tan2 ๐)
= ๐2(sec2 ๐ โ tan2 ๐) โ ๐2(sec2 ๐ โ tan2 ๐)
= ๐2 โ ๐2 [โต sec2 ๐ โ tan2 ๐ = 1]
Hence, ๐ฅ2 โ ๐ฆ2 = ๐2 โ ๐2
3. If sin cos 1x y
a b
and cos sin 1,
x y
a b
prove that
2 2
2 22.
x y
a b
Sol:
We have(๐ฅ
๐sin ๐ โ
๐ฆ
๐cos ๐) = 1
Squaring both side, we have:
(๐ฅ
๐sin ๐ โ
๐ฆ
๐cos ๐)2 = (1)2
โน (๐ฅ2
๐2 sin2 ๐ +๐ฆ2
๐2 cos2 ๐ โ 2๐ฅ
๐ร
๐ฆ
๐sin ๐ cos ๐) = 1 โฆ (๐)
Again, (๐ฅ
๐cos ๐ +
๐ฆ
๐sin ๐) = 1
๐๐๐ข๐๐๐๐๐ ๐๐๐กโ ๐ ๐๐๐, ๐ค๐ ๐๐๐ก:
(๐ฅ
๐cos ๐ +
๐ฆ
๐sin ๐)2 = (1)2
โน(๐ฅ2
๐2 cos2 ๐ +๐ฆ2
๐2 sin2 ๐ + 2๐ฅ
๐ร
๐ฆ
๐sin ๐ cos ๐) = โฆ (๐๐)
Now, adding (i) and (ii), we get:
(๐ฅ2
๐2 sin2 ๐ +๐ฆ2
๐2 cos2 ๐ โ 2๐ฅ
๐ร
๐ฆ
๐sin ๐ cos ๐) + (
๐ฅ2
๐2 cos2 ๐ +๐ฆ2
๐2 sin2 ๐ + 2๐ฅ
๐
๐ฆ
๐sin ๐ cos ๐)
โน๐ฅ2
๐2 sin2 ๐ +๐ฆ2
๐2 cos2 ๐ +๐ฅ2
๐2 cos2 ๐ +๐ฆ2
๐2 sin2 ๐ = 2
โน(๐ฅ2
๐2 sin2 ๐ +๐ฅ2
๐2 cos2 ๐) + (๐ฆ2
๐2 cos2 ๐ +๐ฆ2
๐2 sin2 ๐) = 2
โน๐ฅ2
๐2 (sin2 ๐ + cos2 ๐) +
๐ฆ2
๐2 (cos2 ๐ + sin2 ๐) = 2
โน๐ฅ2
๐2 +๐ฆ2
๐2 = 2 [โต sin2 ๐ + cos2 ๐ = 1]
โด ๐ฅ2
๐2+
๐ฆ2
๐2= 2
4. If sec tan m and sec tan ,n show that 1.mn
Sol:
We have (sec ๐ + tan ๐) = ๐ โฆ (๐)
Again, (sec ๐ โ tan ๐) = ๐ โฆ (๐๐)
Now, multiplying (i) and (ii), we get:
(sec ๐ + tan ๐)ร(sec ๐ โ tan ๐) = ๐๐
Class X Chapter 8 โ Trigonometric Identities Maths
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=> sec2 ๐ โ tan2 ๐ = ๐๐
= > 1 = ๐๐ [โต sec2 ๐ โ tan2 ๐ = 1]
โด ๐๐ =1
5. If cos cotec m and sec cot ,co n show that 1.mn
Sol:
We have (๐๐๐ ๐๐ ๐ + cot ๐) = ๐ โฆ . (๐)
๐ด๐๐๐๐, (๐๐๐ ๐๐ ๐ โ cot ๐) = ๐ โฆ (๐๐)
๐๐๐ค, ๐๐ข๐๐ก๐๐๐๐ฆ๐๐๐ (๐)๐๐๐ (๐๐), ๐ค๐ ๐๐๐ก:
(๐๐๐ ๐๐๐ + cot ๐)ร(๐๐๐ ๐๐๐ โ cot ๐) = ๐๐
= > ๐๐๐ ๐๐2๐ โ cot2 ๐ = ๐๐
= > 1 = ๐๐ [โต ๐๐๐ ๐๐2๐ โ cot2 ๐ = 1]
โด ๐๐ = 1
6. If 3cosx a and 3sin ,y b prove that
2 2
3 3
1.x y
a b
Sol:
๐๐ โ๐๐ฃ๐ ๐ฅ = acos3 ๐
= >๐ฅ
๐= cos3 ๐ โฆ (๐)
๐ด๐๐๐๐, ๐ฆ = ๐๐ ๐๐3๐
= >๐ฆ
๐= sin3 ๐ โฆ (๐๐)
๐๐๐ค, ๐ฟ๐ป๐ = (๐ฅ
๐)
2
3 + (๐ฆ
๐)
2
3
= (cos3 ๐)2
3 + (sin3 ๐)2
3 [๐น๐๐๐ (๐)๐๐๐ (๐๐)]
= cos2 ๐ + sin2 ๐
= 1
๐ป๐๐๐๐, ๐ฟ๐ป๐ = ๐ ๐ป๐
7. If tan sin m and tan sin ,n prove that 2
2 2 16 .m n mn
Sol:
๐๐ โ๐๐ฃ๐ (tan ๐ + sin ๐) = ๐ ๐๐๐ (tan ๐ โ sin ๐) = ๐
๐๐๐ค, ๐ฟ๐ป๐ = (๐2 โ ๐2)2
= [(tan ๐ + sin ๐)2 โ (tan ๐ โ sin ๐)2]2
= [(tan2 ๐ + sin2 ๐ + 2 tan ๐ sin ๐) โ (tan2 ๐ + sin2 ๐ โ 2 tan ๐ sin ๐)]2
= [(tan2 ๐ + sin2 ๐ + 2 tan ๐ sin ๐ โ tan2 ๐ โ sin2 ๐ + 2 tan ๐ sin ๐)]2
= (4 tan ๐ sin ๐)2
= 16 tan2 ๐ sin2 ๐
Class X Chapter 8 โ Trigonometric Identities Maths
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= 16sin2 ๐
cos2 ๐sin2 ๐
= 16(1โcos2 ๐) sin2 ๐
cos2 ๐
= 16[tan2 ๐(1 โ cos2 ๐)]
= 16(tan2 ๐ โ tan2 ๐ cos2 ๐)
= 16(tan2 ๐ โsin2 ๐
cos2 ๐ร cos2 ๐)s
= 16(tan2 ๐ โ sin2 ๐)
= 16(tan ๐ + sin ๐)(tan ๐ โ sin ๐)
= 16 ๐๐ [(tan ๐ + sin ๐)(tan ๐ โ sin ๐) = ๐๐]
โด (๐2 โ ๐2)(๐2 โ ๐2)2 = 16๐๐
8. If cot tan m and sec cos n prove that 2 2
2 23 3( ) ( ) 1m n mn
Sol:
๐๐ โ๐๐ฃ๐ (cot ๐ + tan ๐) = ๐ ๐๐๐ (sec ๐ โ cos ๐) = ๐
Now, ๐2๐ = [(cot ๐ + tan ๐)2 (sec ๐ โ cos ๐)]
= [(1
tan ๐+ tan ๐)2 (
1
cos ๐โ cos ๐)]
= (1+tan2 ๐)2
tan2 ๐ร
(1โcos2 ๐)
cos ๐
= sec4 ๐
tan2 ๐ร
sin2 ๐
cos ๐
= sec4 ๐
sin2 ๐
cos2 ๐
รsin2 ๐
cos ๐
= cos2 ๐ร sec4 ๐
cos ๐
= cos ๐ sec4 ๐
= 1
sec ๐ร sec4 ๐ = sec3 ๐
โด (๐2๐)2
3 = (sec3 ๐)2
3 = sec2 ๐
๐ด๐๐๐๐, ๐๐2 = [(cot ๐ + tan ๐)(sec ๐ โ cos ๐)2]
= [(1
tan ๐+ tan ๐). (
1
cos ๐โ cos ๐)2]
= (1+tan2 ๐)
tan ๐ร
(1โcos2 ๐)2
cos2 ๐
= sec2 ๐
tan ๐ร
sin4 ๐
cos2 ๐
= sec2 ๐
sin ๐
cos ๐
รsin4 ๐
cos2 ๐
= sec2 ๐ร sin3 ๐
cos ๐
= 1
cos2 ๐ร
sec3 ๐
cos ๐= tan3 ๐
โด (๐๐2)2
3 = (tan3 ๐)2
3 = tan2 ๐
Class X Chapter 8 โ Trigonometric Identities Maths
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๐๐๐ค, (๐2๐)2
3 โ (๐๐2)2
3
= sec2 ๐ โ tan2 ๐ = 1
= RHS
Hence proved.
9. If 3 3(cos sin ) (sec cos ) ,ec a and b prove that 2 2 2 2( ) 1a b a b
Sol:
๐๐ โ๐๐ฃ๐ (๐๐๐ ๐๐ ๐ โ sin ๐) = ๐3
= > ๐3 = (1
sin ๐โ sin ๐)
= > ๐3 =(1โsin2 ๐)
sin ๐=
cos2 ๐
sin ๐
โด
2
3
1
3
cosa=
sin
๐ด๐๐๐๐, (sec ๐ โ cos ๐) = ๐3
= > ๐3 = (1
cos ๐โ cos ๐)
= (1โcos2 ๐)
cos ๐
= sin2 ๐
cos ๐
โด ๐ =sin
23 ๐
cos13 ๐
๐๐๐ค, ๐ฟ๐ป๐ = ๐2๐2(๐2 + ๐2)
= ๐4๐2 + ๐2๐4
= ๐3(๐๐2) + (๐2๐2)๐3
= cos2 ๐
sin ๐ร [
cos23 ๐
sin13 ๐
รsin
43 ๐
cos23 ๐
] + [cos
43 ๐
sin23 ๐
รsin
23 ๐
cos13 ๐
] รsin2 ๐
cos ๐
= cos2 ๐
sin ๐ร sin ๐ + cos ๐ ร
sin2 ๐
cos ๐
= cos2 ๐ + sin2 ๐ = 1
= RHS
Hence, proved.
10. If 2sin 3cos 2, prove that 3sin 2cos 3.
Sol:
๐บ๐๐ฃ๐๐, (2 sin ๐ + 3 cos ๐) = 2 โฆ (๐)
๐๐ โ๐๐ฃ๐ (2 sin ๐ + 3 cos ๐)2 + (3 sin ๐ โ 2 cos ๐)2
= 4 sin2 ๐ + 9 cos2 ๐ + 12 sin ๐ cos ๐ + 9 sin2 ๐ + 4 cos2 ๐ โ 12 sin ๐ cos ๐
= 4(sin2 ๐ + cos2 ๐) + 9(sin2 ๐ + cos2 ๐) = 4+9
= 13
Class X Chapter 8 โ Trigonometric Identities Maths
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i.e., (2 sin ๐ + 3 cos ๐)2 + (3 sin ๐ โ 2 cos ๐)2 = 13
= > 22 + (3 sin ๐ โ 2 cos ๐)2 = 13
= > (3 sin ๐ โ 2 cos ๐)2 = 13 โ 4
= > (3 sin ๐ โ 2 cos ๐)2 = 9
= > (3 sin ๐ โ 2 cos ๐) = ยฑ3
11. If sin cos 2, prove that cot 2 1 .
Sol:
๐๐ โ๐๐ฃ๐, (sin ๐ + cos ๐) = โ2 cos ๐
๐ท๐๐ฃ๐๐๐๐๐ ๐๐๐กโ ๐ ๐๐๐๐ ๐๐ฆ sin ๐ , ๐ค๐ ๐๐๐ก
Sin ๐
sin ๐+
cos ๐
sin ๐=
โ2 cos ๐
sin ๐
โน 1 + cot ๐ = โ2 cot ๐
โน โ2 cot ๐ โ cot ๐ = 1
โน (โ2 โ 1) cot ๐ = 1
โน cot ๐ =1
(โ2โ1)
โน cot ๐ =1
(โ2โ1ร
(โ2+1)
(โ2+1)
โน cot ๐ =(โ2+1)
2โ1
โน cot ๐ =(โ2+1)
1
โด cot ๐ = (โ2 + 1)
12. If (cos sin ) 2 sin , prove that (sin cos ) 2 cos .
Sol:
๐บ๐๐ฃ๐๐: cos ๐ + sin ๐ = โ2 sin ๐
๐๐ โ๐๐ฃ๐ (sin ๐ + cos ๐)2 + (sin ๐ โ cos ๐)2 = 2(sin2 ๐ + cos2 ๐)
= > (โ2 sin ๐)2
+ (sin ๐ โ cos ๐)2 = 2
= > 2 sin2 ๐ + (sin ๐ โ cos ๐)2 = 2
= > (sin ๐ โ cos ๐)2 = 2 โ 2 sin2 ๐
= > (sin ๐ โ cos ๐)2 = 2(1 โ sin2 ๐)
= > (sin ๐ โ cos ๐)2 = 2 cos2 ๐
= > (sin ๐ โ cos ๐) = โ2 cos ๐
Hence proved.
13. If sec tan p , prove that
(i) 1 1
sec2
pp
(ii) 1 1
tan2
pp
(iii) 2
2
1in
1
ps
p
Class X Chapter 8 โ Trigonometric Identities Maths
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Sol:
(i) We have, sec tan p โฆโฆโฆโฆโฆโฆโฆโฆโฆโฆ..(1)
sec tan sec tan
1 sec tanp
2 2sec tan
sec tanp
1
sec tan
1sec tan ..........................(2)
p
p
Adding (1) and (2), we get
12sec
1 1sec
2
pp
pp
(ii) Subtracting (2) from (1), we get
12 tan
1 1tan
2
pp
pp
(iii) Using (i) and (ii), we get
tansin
sec
1 1
2
1 1
2
pp
pp
2
2
1
1
p
p
p
p
2
2
1sin
1
p
p
14. If tan A = n tan B and sin A =m sin B, prove that
2
2
2
1cos
1
mA
n
.
Sol:
We have tan tanA n B
Class X Chapter 8 โ Trigonometric Identities Maths
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cot ......( )tan
nB i
A
Again, sin A = m sin B
cos ........( )sin
mecB ii
A
Squaring (i) and (ii) and subtracting (ii) form (i), we get
2 2
2 2
2 2cos cot
sin tan
m nec B B
A A
2 2
2 2
cos1
sin sin
m n
A A
2 2 2 2cos sinm n A A 2 2 2 2cos 1 cosm n A A
2 2 2 2cos cos 1n A A m
2 2 2
2
2
2
cos 1 1
1cos
1
A n m
mA
n
2
2
2
1cos
1
mA
n
15. 15. if (cos sin )m and (cos sin )n then show that 2
2
1 tan
m n
n m
.
Sol:
m nLHS
n m
m n
n m
m n
mn
(cos sin ) (cos sin )
cos sin cos sin
2 2
2cos
cos sin
2 2
2cos
cos sin
Class X Chapter 8 โ Trigonometric Identities Maths
______________________________________________________________________________
2 2
2cos
cos
cos sin
cos
2 2
2 2
2
cos sin
cos cos
2
2
1 tan
RHS
Exercise โ 8C
1. Write the value of 2 21 sin sec .
Sol:
(1 โ sin2 ๐) sec2 ๐
= cos2 ๐ ร1
cos2 ๐
= 1
2. Write the value of 2 21 cos secco .
Sol:
(1 โ cos2 ๐)๐๐๐ ๐๐2 ๐
= sin2 ๐ร1
sin2 ๐
= 1
3. Write the value of 2 21 tan cos .
Sol:
(1 + tan2 ๐) cos2 ๐
= sec2 ๐ร1
sec2 ๐
= 1
4. Write the value of 2 21 cot sin .
Sol:
= (1 + cot2 ๐) sin2 ๐
= ๐๐๐ ๐๐2๐ร1
๐๐๐ ๐๐2๐
= 1
Class X Chapter 8 โ Trigonometric Identities Maths
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5. Write the value of 2
2
1sin
1 tan
.
Sol:
(sin2 ๐ +1
1+tan2 ๐)
= (sin2 ๐ +1
sec2 ๐)
= (sin2 ๐ + cos2 ๐)
= 1
6. Write the value of 2
2
1cot .
sin
Sol:
(cot2 ๐ โ1
sin2 ๐)
= (cot2 ๐ โ ๐๐๐ ๐๐2๐)
= -1
7. Write the value of sin cos 90 cos sin 90 .
Sol:
Sin ๐ cos(900 โ ๐) + cos ๐ sin(900 โ ๐)
= sin ๐ sin ๐ + cos ๐ cos ๐
= sin2 ๐ + cos2 ๐
= 1
8. Write the value of 2 2cosec 90 tan .
Sol:
๐๐๐ ๐๐2(900 โ ๐) โ tan2 ๐
= sec2 ๐ โ tan2 ๐
= 1
9. Write the value of 2sec 1 sin (1 sin ) .
Sol:
Sec2 ๐(1 + sin ๐)(1 โ sin ๐)
= sec2 ๐(1 โ sin2 ๐)
= 1
cos2 ๐ร cos2 ๐
= 1
Class X Chapter 8 โ Trigonometric Identities Maths
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10. Write the value of 2sec 1 cos (1 cos ).co
Sol:
๐๐๐ ๐๐2๐(1 + cos ๐)(1 โ cos ๐)
= ๐๐๐ ๐๐2๐(1 โ cos2 ๐)
= 1
sin2 ๐ร sin2 ๐
= 1
11. Write the value of 2 2 2 2sin cos 1 tan (1 cot ).
Sol:
Sin2 ๐ cos2 ๐(1 + tan2 ๐)(1 + cot2 ๐)
= sin2 ๐ cos2 ๐ sec2 ๐๐๐๐ ๐๐2๐
= sin2 ๐ร cos2 ๐ร1
cos2 ๐ร
1
sin2 ๐
= 1
12. Write the value of 21 tan (1 sin )(1 sin ).
Sol:
(1 + tan2 ๐) (1 + sin ๐)(1 โ sin ๐)
= sec2 ๐(1 โ sin2 ๐)
= 1
cos2 ๐ร cos2 ๐
= 1
13. Write the value of 2 23cot 3cos .ec
Sol:
3 cot2 ๐ โ 3๐๐๐ ๐๐2๐
= 3(cot2 ๐ โ ๐๐๐ ๐๐2๐)
= 3(โ1)
= -3
14. Write the value of 2
2
44 tan .
cos
Sol:
4 tan2 ๐ โ4
cos2 ๐
= 4 tan2 ๐ โ 4 sec2 ๐
= 4(tan2 ๐ โ sec2 ๐)
= 4(โ1)
= -4
Class X Chapter 8 โ Trigonometric Identities Maths
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15. Write the value of 2 2
2 2
tan sec.
cot cosec
Sol:
tan2 ๐โsec2 ๐
cot2 ๐โ๐๐๐ ๐๐2๐
= โ1
โ1
= 1
16. If 1
sin ,2
write the value of 23cot 3 .
Sol:
๐ด๐ , sin ๐ =1
2
๐๐, ๐๐๐ ๐๐๐ =1
sin ๐= 2 โฆ . . (๐)
Now,
3 cot2 ๐ + 3
= 3(cot2 ๐ + 1)
= 3๐๐๐ ๐๐2๐
= 3(2)2 [๐๐ ๐๐๐ (๐)]
= 3(4)
= 12
17. If 2
cos3
, write the value of 24 4 tan .
Sol:
4 + 4 tan2 ๐
= 4(1 + tan2 ๐)
= 4 sec2 ๐
= 4
cos2 ๐
= 4
(2
3)
2
= 4
(4
9)
= 4ร9
4
= 9
18. If 7
cos ,25
write the value of tan cot .
Sol:
๐ด๐ sin2 ๐ = 1 โ cos2 ๐
Class X Chapter 8 โ Trigonometric Identities Maths
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= 1 โ (7
25)
2
= 1 โ49
625
= 625โ49
625
โน sin2 ๐ =576
625
โน sin ๐ = โ576
625
โน sin ๐ =24
25
Now,
tan ๐ + cot ๐
= sin ๐
cos ๐+
cos ๐
sin ๐
= sin2 ๐+cos2 ๐
cos ๐ sin ๐
= 1
(7
25ร
24
25)
= 1
(168
625)
= 625
168
19. If 2
cos ,3
write the value of
sec 1.
sec 1
Sol: Sec ๐โ1
sec ๐+1
= (
1
cos ๐โ
1
1)
(1
cos ๐+
1
1)
= (
1โcos ๐
cos ๐)
(1+cos ๐
cos ๐)
= 1โcos ๐
1+cos ๐
= (
1
1โ
2
3)
(1
1+
2
3)
= (
1
3)
(5
3)
= 1
5
Class X Chapter 8 โ Trigonometric Identities Maths
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20. If 5 tan 4 , write the value of
cos sin.
cos sin
Sol:
๐๐ โ๐๐ฃ๐,
5 tan ๐ = 4
โนtan ๐ =4
5
Now, (cos ๐โsin ๐)
(cos ๐+sin ๐)
= (
cos ๐
cos ๐โ
sin ๐
cos ๐)
(cos ๐
cos ๐+
sin ๐
cos ๐) (๐ท๐๐ฃ๐๐๐๐๐ ๐๐ข๐๐๐๐๐ก๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐ก๐๐ ๐๐ฆ cos ๐)
= (1โtan ๐)
(1+tan ๐)
= (
1
1โ
4
5)
(1
1+
4
5)
= (
1
5)
(9
5)
= 1
9
21. If 3cot 4 , write the value of
2cos sin.
4cos sin
Sol:
๐๐ โ๐๐ฃ๐, 3 cot ๐ = 4
โน cot ๐ =4
3
Now, (2 cos ๐+sin ๐)
(4 cos ๐โsin ๐)
= (
2 cos ๐
sin ๐+
sin ๐
sin ๐)
(4 cos ๐
sin ๐โ
sin ๐
sin ๐) (๐ท๐๐ฃ๐๐๐๐๐ ๐๐ข๐๐๐๐๐ก๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐ก๐๐ ๐๐ฆ sin ๐)
= (2 cot ๐+1)
(4 cot ๐โ1)
= (2ร
4
3+1)
(4ร4
3โ1)
= (
8
3+
1
1)
(16
3โ
1
1)
= (
8+3
3)
(16โ3
3)
= (
11
3)
(13
3)
Class X Chapter 8 โ Trigonometric Identities Maths
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= 11
13
22. If 1
cot ,3
write the value of
2
2
1 cos.
2 sin
Sol:
๐๐ โ๐๐ฃ๐,
Cot ๐ =1
โ3
โน cot ๐ = cot (๐
3)
โน ๐ =๐
3
Now,
(1โcos2 ๐)
(2โsin2 ๐)
= 1โcos2(
๐
3)
2โsin2(๐
3)
= 1โ(
1
2)
2
2โ(โ3
2)
2
= (
1
1โ
1
4)
(2
1โ
3
4)
= (
3
4)
(5
4)
= 3
5
23. If 1
tan ,5
write the value of
2 2
2 2
cos sec.
cos sec
ec
ec
Sol:
(๐๐๐ ๐๐2๐โsec2 ๐)
(๐๐๐ ๐๐2๐+sec2 ๐)
= (1+cot2 ๐)โ(1+tan2 ๐)
(1+cot2 ๐)+(1+tan2 ๐)
= (1+
1
tan2 ๐)โ(1+tan2 ๐)
(1+1
tan2 ๐)+(1+tan2 ๐)
= (1+
1
tan2 ๐โ1โtan2 ๐)
(1+1
tan2 ๐+1+tan2 ๐)
= (
1
tan2 ๐โtan2 ๐)
(1
tan2 ๐+tan2 ๐+2)
Class X Chapter 8 โ Trigonometric Identities Maths
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= (
โ5
1)
2
โ(1
โ5)
2
(โ5
1)
2
+(1
โ5)
2+2
= (
5
1โ
1
5)
(5
1+
1
5+
2
1)
= (
24
5)
(36
5)
= 24
36
= 2
3
24. If 4
cot 903
A and A B , what is the value of tan B?
Sol:
๐๐ โ๐๐ฃ๐,
๐๐๐ก๐ด =4
3
โนcot(900 โ ๐ต) =4
3 (๐ด๐ , ๐ด + ๐ต = 900)
โด ๐ก๐๐๐ต =4
3
25. If 3
cos 905
B and A B , find the value of sin A.
Sol:
๐๐ โ๐๐ฃ๐,
๐๐๐ ๐ต =3
5
โน cos(900 โ ๐ด) =3
5 (๐ด๐ , ๐ด + ๐ต = 900)
โด ๐ ๐๐๐ด =3
5
26. If 3 sin cos and is an acute angle, find the value of .
Sol:
๐๐ โ๐๐ฃ๐,
โ3 sin ๐ = cos ๐
โน sin ๐
cos ๐=
1
โ3
โน tan ๐ =1
โ3
โน tan ๐ = ๐ก๐๐300
โด ๐ = 300
Class X Chapter 8 โ Trigonometric Identities Maths
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27. Write the value of tan10 tan 20 tan 70 tan 80 .
Sol:
๐ก๐๐100 ๐ก๐๐200 ๐ก๐๐700 ๐ก๐๐800
= cot(900 โ 100) cot(900 โ 200) ๐ก๐๐700 ๐ก๐๐800
= ๐๐๐ก800 ๐๐๐ก700 ๐ก๐๐700 ๐ก๐๐800
= 1
๐ก๐๐ 800ร
1
tan 700ร tan 700 ร tan 800
= 1
28. Write the value of tan1 tan 2 ........ tan 89 .
Sol:
Tan 10 tan 20 โฆ tan 890
= tan 10 tan 20 tan 30 โฆ tan 450 โฆ tan 870 tan 880 tan 890
= tan 10 tan 20 tan 30 โฆ tan 450 โฆ cot(900 โ 870) cot(900 โ 880) cot(900 โ 890)
= tan 10 tan 20 tan 30 โฆ tan 450 โฆ cot 30 cot 20 cot 10
= tan 10 ร tan 20 ร tan 30ร โฆร1ร โฆร 1
tan 30 ร1
tan 20 ร1
tan 10
= 1
29. Write the value of cos1 cos 2 ........cos180 .
Sol:
Cos 10 cos 20 โฆ cos 1800
= cos 10 cos 20 โฆ cos 900 โฆ cos 1800
= cos 10 cos 20 โฆ 0 โฆ cos 1800
= 0
30. If 5
tan ,12
A find the value of sin cos sec .A A A
Sol:
(๐ ๐๐๐ด + ๐๐๐ ๐ด)๐ ๐๐๐ด
= (๐ ๐๐๐ด + ๐๐๐ ๐ด) 1
๐๐๐ ๐ด
= ๐ ๐๐๐ด
๐๐๐ ๐ด+
๐๐๐ ๐ด
๐๐๐ ๐ด
= ๐ก๐๐๐ด + 1
= 5
12+
1
1
= 5+12
12
= 17
12
31. If sin cos 45 , where is acute, find the value of .
Sol:
๐๐ โ๐๐ฃ๐,
Sin ๐ = cos(๐ โ 450)
โน cos(900 โ ๐) = cos(๐ โ 450)
Comparing both sides, we get
Class X Chapter 8 โ Trigonometric Identities Maths
______________________________________________________________________________
900 โ ๐ = ๐ โ 450
โน ๐ + ๐ = 900 + 450
โน 2๐ = 1350
โน ๐ = (135
2)
0
โด ๐ = 67.50
32. Find the value of sin 50 cos 40
4cos50 cos 40 .cos 40 sec50
ecec
Sol:
Sin 500
cos 400 +๐๐๐ ๐๐400
sec 500 โ 4 cos 500 ๐๐๐ ๐๐ 400
= cos(900โ500)
cos 400 +sec(900โ400)
sec 500 โ 4 sin(900 โ 500) ๐๐๐ ๐๐ 400
= ๐๐๐ 400
cos 400 +sec 500
sec 500 โ 4 sin 400 ร1
sin 400
= 1 + 1 โ 4
= โ2
33. Find the value of sin 48 sec 42 cos 48 cos 42ec .
Sol:
Sin 480 sec 420 + cos 480 ๐๐๐ ๐๐ 420
= sin 480 ๐๐๐ ๐๐(900 โ 420) + cos 480 sec(900 โ 420)
= sin 480 ๐๐๐ ๐๐ 480 + cos 480 sec 480
= sin 480ร 1
sin 480 + cos 480 ร1
cos 480
= 1+1
=2
34. If sin cos ,x a and y b write the value of 2 2 2 2 .b x a y
Sol:
(๐2๐ฅ2 + ๐2๐ฆ2)
= ๐2(asin ๐)2 + ๐2(๐๐๐๐ ๐)2
= ๐2๐2 sin2 ๐ + ๐2๐2 cos2 ๐
= ๐2๐2(sin2 ๐ + cos2 ๐)
= ๐2๐2(1)
= ๐2๐2
35. If 5
5 sec tan ,x andx
find the value of 2
2
15 .x
x
Sol:
5 (๐ฅ2 โ1
๐ฅ2)
= 25
5(๐ฅ2 โ
1
๐ฅ2)
Class X Chapter 8 โ Trigonometric Identities Maths
______________________________________________________________________________
= 1
5(25๐ฅ2 โ
25
๐ฅ2)
= 1
5 [(5๐ฅ)2 โ (
5
๐ฅ)
2
]
= 1
5 [(๐ ๐๐ ๐)2 โ (๐ก๐๐ ๐)2]
= 1
5 (๐ ๐๐2 ๐ โ ๐ก๐๐2 ๐)
= 1
5 (1)
= 1
5
36. If 2
cos 2 cot ,ec x andx
find the value of 2
2
12 .x
x
Sol:
2 (๐ฅ2 โ1
๐ฅ2)
= 4
2 (๐ฅ2 โ
1
๐ฅ2)
= 1
2 (4๐ฅ2 โ
4
๐ฅ2)
= 1
2 [(2๐ฅ)2 โ (
2
๐ฅ)
2
]
= 1
2 [(๐๐๐ ๐๐ ๐)2 โ (sec ๐)2]
= 1
2 (๐๐๐ ๐๐2๐ โ sec2 ๐)
= 1
2 (1)
= 1
2
37. If tansec x , find the value of sec .
Sol:
๐๐ โ๐๐ฃ๐,
Sec ๐ + tan ๐ = ๐ฅ โฆ โฆ . (๐)
โนsec ๐+tan ๐
1ร
sec ๐โtan ๐
sec ๐โtan ๐= ๐ฅ
โน sec2 ๐โtan2 ๐
sec ๐โtan ๐= ๐ฅ
โน 1
sec ๐โtan ๐=
๐ฅ
1
โน sec ๐ โ tan ๐ =1
๐ฅ โฆ . . (๐๐)
๐ด๐๐๐๐๐ (๐)๐๐๐ (๐๐), ๐ค๐ ๐๐๐ก
2 sec ๐ = ๐ฅ +1
๐ฅ
โน 2 sec ๐ =๐ฅ2+1
๐ฅ
โด sec ๐ =๐ฅ2+1
2๐ฅ
Class X Chapter 8 โ Trigonometric Identities Maths
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38. Find the value of cos38 cos 52
tan18 tan 35 tan 60 tan 72 tan 55
ec
.
Sol:
Cos 380 ๐๐๐ ๐๐ 520
tan 180 tan 350 tan 600 tan 720 tan 550
= cos 380 sec(900โ520)
cot(900โ180) cot(900โ350) tan 600 tan 720 tan 550
= cos 380 sec 380
cot 720 cot 550 tan 600 tan 720 tan 550
= cos 380ร
1
cos 3801
tan 720ร1
tan 550รโ3ร tan 720ร tan 550
= 1
โ3
39. If sin x , write the value of cot .
Sol:
Cot ๐ =cos ๐
sin ๐
= โ1โsin2 ๐
sin ๐
= โ1โ๐ฅ2
2
40. If sec x , write the value of tan .
Sol:
As, tan2 ๐ = sec2 ๐ โ 1
So, tan ๐ = โsec2 ๐ โ 1 = โ๐ฅ2 โ 1
Formative Assessment
1.
2 22 2
2 2
cos 56 cos 343tan 56 tan 34 ?
sin 56 34sin
(a) 1
32
(b) 4
(c) 6 (d) 5
Answer: (b) 4
Sol:
cos2 560+cos2 340
sin2 560+sin2 340 + 3 tan2 560 tan2 340\
={cos(900โ340)}
2+cos2 340
{sin(900โ340)}2+sin2 340 + 3{tan(900 โ 340)}2 tan2 340
=sin2 340+cos2 340
cos2 340+sin2 340 + 3 cot2 340 tan2 340
cos 90 sin ,sin 90
cos tan 90 cotand
Class X Chapter 8 โ Trigonometric Identities Maths
______________________________________________________________________________
=1
1+ 3ร1 [โต cot ๐ =
1
tan ๐ ๐๐๐ sin2 ๐ + cos2 ๐ = 1]
= 4
2. The value of 2 2 2 2 21 1sin 30 cos 45 4 tan 30 sin 90 cot 60 ?
2 8
(a) 3
8 (b)
5
8
(c) 6 (d) 2
Answer: (d) 2
Sol:
(sin2 300 cos2 450) + 4 tan2 300 +1
2sin2 900 +
1
8cot2 600
=1
22 ร1
(โ2)2 + 4ร
1
(โ3)2 +
1
2ร12 +
1
8 ร
1
(โ3)2
1 1sin 30 cos 45
2 2
1 1tan 30 cot 60
2 3
and
and and
=1
4ร
1
2+ 4ร
1
3+
1
2+
1
24
=1
8+
4
3+
1
2+
1
24
=3+32+12+1
24
=48
24
= 2
3. If 2cos cos 1A A then 2 4sin sin ?A A
(a) 1
2 (b) 2
(c) 1 (d) 4
Answer: (c) 1
Sol: 2cos 1A A
=> cos ๐ด = sin2 ๐ด โฆ (๐)
๐๐๐ข๐๐๐๐๐ ๐๐๐กโ ๐ ๐๐๐๐ ๐๐ (๐), ๐ค๐ ๐๐๐ก:
cos2 ๐ด = sin4 ๐ด โฆ (๐๐)
๐ด๐๐๐๐๐ (๐)๐๐๐ (๐๐), ๐ค๐ ๐๐๐ก:
sin2 ๐ด + sin4 ๐ด = cos ๐ด + cos2 ๐ด
=> sin2 ๐ด + sin4 ๐ด = 1 [โต cos ๐ด + cos2 ๐ด = 1]
Class X Chapter 8 โ Trigonometric Identities Maths
______________________________________________________________________________
4. If 3
sin2
then cos cot ?ec
(a) 2 3 (b) 2 3
(c) 2 (d) 3
Answer: (d) 3
Sol:
๐บ๐๐ฃ๐๐: sin ๐ =โ3
2 ๐๐๐ cos ๐๐๐ =
2
โ3
cos ๐๐2๐ โ cot2 ๐ = 1
=> cot2 ๐ = cos ๐๐2๐ โ 1
=> cot2 ๐ =4
3โ 1 [๐บ๐๐ฃ๐๐]
=> cot ๐ =1
โ3
โดcos ๐๐๐ + cot ๐ =2
โ3+
1
โ3
= 3
โ3
=โ3รโ3
โ3
=โ3
5. If 4
cot ,5
A prove that
sin cos9.
sin cos
A A
A A
Sol:
๐บ๐๐ฃ๐๐ โถ cot ๐ด =4
5
๐๐๐๐ก๐๐๐ cot ๐ด =cos ๐ด
sin ๐ด ๐๐๐ ๐ ๐๐๐ข๐๐๐๐ ๐กโ๐ ๐๐๐ข๐๐ก๐๐๐, ๐ค๐ ๐๐๐ก โถ
cos2 ๐ด
sin2 ๐ด=
16
25
=> 25 cos2 ๐ด = 16 sin2 ๐ด
=> 25 cos2 ๐ด = 16 โ 16 cos2 ๐ด
=> cos2 ๐ด =16
41
=> cos ๐ด =4
โ41
โดsin2 ๐ด = 1 โ cos2 ๐ด
=1 โ16
41
๐๐๐ค, sin ๐ด = โ25
41
=> sin ๐ด =5
โ41
โด ๐ฟ๐ป๐ =sin ๐ด+cos ๐ด
sin ๐ดโcos ๐ด
Class X Chapter 8 โ Trigonometric Identities Maths
______________________________________________________________________________
=
5
โ41+
4
โ415
โ41โ
4
โ41
=9
1
= 9 = ๐ ๐ป๐
6. If 2
2 sec tan ,x Aand Ax
prove that 2
2
1 1.
4x
x
Sol:
๐บ๐๐ฃ๐๐: 2๐ฅ = sec ๐ด
=> ๐ฅ =sec ๐ด
2 โฆ (๐)
๐๐๐2
๐ฅ= tan ๐ด
=>1
๐ฅ= tan ๐ด โฆ (๐๐)
โด๐ฅ +1
๐ฅ=
sec ๐ด
2+
tan ๐ด
2 [โต ๐น๐๐๐ (๐)๐๐๐ (๐๐)]
๐ด๐๐ ๐, ๐ฅ โ1
๐ฅ=
sec ๐ด
2โ
tan ๐ด
2
โด (๐ฅ +1
๐ฅ) (๐ฅ โ
1
๐ฅ) = (
sec ๐ด
2+
tan ๐ด
2) (
sec ๐ด
2โ
tan ๐ด
2)
=> ๐ฅ2 โ1
๐ฅ2 =1
4 (sec2 ๐ด โ tan2 ๐ด)
โด ๐ฅ2 โ1
๐ฅ2 =1
4ร1 (โต sec2 ๐ด โ tan2 ๐ด = 1)
=1
4
๐ป๐๐๐๐ ๐๐๐๐ฃ๐๐.
7. If 3 tan 3sin , prove that 2 2sin cos 1
.3
Sol:
๐บ๐๐ฃ๐๐: โ3 tan ๐ = 3 sin ๐
=>โ3
cos ๐= 3 [โต tan ๐ =
sin ๐
cos ๐]
=> cos ๐ =โ3
3
=> cos2 ๐ =3
9
โดsin2 ๐ = 1 โ3
9
=> sin2 ๐ =6
9
โด๐ฟ๐ป๐ = sin2 ๐ โ cos2 ๐
=6
9โ
3
9 [โด sin2 ๐ =
6
9, cos2 ๐ =
3
9]
=3
9
=1
3
Class X Chapter 8 โ Trigonometric Identities Maths
______________________________________________________________________________
=๐ ๐ป๐
๐ป๐๐๐๐ ๐๐๐๐ฃ๐๐.
8. Prove that
2 2
2 2
sin 73 sin 171.
cos 28 cos 62
Sol: (sin2 730+sin2 170)
(cos2 280+cos2 620)= 1.
๐ฟ๐ป๐ =sin2 730+sin2 170
cos2 280+cos2 620
=[sin(900โ170)]
2+sin2 170
[cos(900โ620)]2+cos2 620
=cos2 170+sin2 170
sin2 620+cos2 620
=1
1 [โต sin2 ๐ + cos2 ๐ = 1]
= 1 = ๐ ๐ป๐
9. If 2sin 2 3 , prove that 30 .
Sol:
2 sin(2๐) = โ3
=> sin(2๐) =โ3
2
=> sin(2๐) = sin(600)
=> 2๐ = 600
=> ๐ =600
2
=> ๐ = 300
10. Prove that 1 cos
cos cot .1 cos
Aec A A
A
Sol:
โ1+cos ๐ด
1โcos ๐ด= (๐๐๐ ๐๐๐ด + cot ๐ด).
๐ฟ๐ป๐ = โ1+cos ๐ด
1โcos ๐ด
๐๐ข๐๐ก๐๐๐๐ฆ๐๐๐ ๐กโ๐ ๐๐ข๐๐๐๐๐ก๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐ก๐๐ ๐๐ฆ (1 + cos ๐ด), ๐ค๐ โ๐๐ฃ๐:
โ(1+cos ๐ด)2
(1โcos ๐ด)(1+cos ๐ด)
= โ(1+cos ๐ด)2
1 cos2 ๐ด
=1+cos ๐ด
โsin2 ๐ด
=1+cos ๐ด
sin ๐ด
Class X Chapter 8 โ Trigonometric Identities Maths
______________________________________________________________________________
=1
sin ๐ด+
cos ๐ด
sin ๐ด
= ๐๐๐ ๐๐๐ด + cot ๐ด = ๐ ๐ป๐
๐ป๐๐๐๐ ๐๐๐๐ฃ๐๐.
11. If cos cot ,ec p prove that
2
2
1cos
1
p
p
.
Sol:
cos ๐๐๐ + cot ๐ = ๐
=>1
sin ๐+
cos ๐
sin ๐= ๐
=>1+cos ๐
sin ๐= ๐
๐๐๐ข๐๐๐๐๐ ๐๐๐กโ ๐ ๐๐๐๐ , ๐ค๐ ๐๐๐ก:
(1+cos ๐
sin ๐)
2
= ๐2
=>(1+cos ๐)2
sin2 ๐= ๐2
=>(1+cos ๐)2
1โcos2 ๐= ๐2
=>(1+cos ๐)2
(1+cos ๐)(1โcos ๐)= ๐2
=>(1+cos ๐)
(1โcos ๐)= ๐2
=> 1 + cos ๐ = ๐2(1 โ cos ๐)
=1 + cos ๐ = ๐2 โ ๐2 cos ๐
=> cos ๐(1 + ๐2) = ๐2 โ 1
=> cos ๐ =๐2โ1
๐2+1
๐ป๐๐๐๐ ๐๐๐๐ฃ๐๐.
12. Prove that
2 1 cos
cos cot .1 cos
Aec A A
A
Sol:
(๐๐๐ ๐๐๐ด โ cot ๐ด)2 =(1โcos ๐ด)
(1+cos ๐ด).
๐ฟ๐ป๐ = (๐๐๐ ๐๐๐ด โ cot ๐ด)2
= (1
sin ๐ดโ
cos ๐ด
sin ๐ด)
2
= (1โcos ๐ด
sin ๐ด)
2
=(1โcos ๐ด)2
sin2 ๐ด
=(1โcos ๐ด)2
1โcos2 ๐ด [โต sin2 ๐ + cos2 ๐ = 1]
=(1โcos ๐ด)(1โcos ๐ด)
(1โcos ๐ด)(1+cos ๐ด)
=(1โcos ๐ด)
(1+cos ๐ด)= ๐ ๐ป๐
๐ป๐๐๐๐ ๐๐๐๐ฃ๐๐.
Class X Chapter 8 โ Trigonometric Identities Maths
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13. If 5cot 3, show that the value of 5sin 3cos
4sin 3cos
is
16.
29
Sol:
๐บ๐๐ฃ๐๐: 5 cot ๐ = 3
=>5 cos ๐
sin ๐= 3 [โต cot ๐ =
cos ๐
sin ๐]
=> 5 cos ๐ = 3 sin ๐
๐๐๐ข๐๐๐๐๐ ๐๐๐กโ ๐ ๐๐๐๐ , ๐ค๐ ๐๐๐ก: 25 cos2 ๐ = 9 sin2 ๐
=> 25 cos2 ๐ = 9 โ 9 cos2 ๐ [โต sin2 ๐ + cos2 ๐ = 1] => 34 cos2 ๐ = 9
=> cos ๐ = โ9
34
=> cos ๐ =3
โ34
๐ด๐๐๐๐, sin2 ๐ = 1 โ cos2 ๐
=> sin2 ๐ =34โ9
34=
25
34
=> sin ๐ =5
โ34
โด๐ฟ๐ป๐ = (5 sin ๐โ3 cos ๐
4 sin ๐+3 cos ๐)
=5ร
5
โ34โ3ร
3
โ34
4ร5
โ34+3ร
3
โ34
[โต cos ๐ =3
โ34, sin ๐ =
5
โ34]
=25โ9
20+9
=16
29
14. Prove that sin32 cos58 cos32 sin58 1 .
Sol:
(sin 320 cos 580 + cos 320 sin 580) = 1
๐ฟ๐ป๐ = sin 320 cos 580 + cos 320 sin 580
= sin(900 โ 580) cos 580 + cos(900 โ 580) sin 580
2 2
2 2
sin 90 cos ,cos58 cos58 sin 58 sin 58
cos 90 cos
cos 58 sin 58
1 sin cos 1
RHS
Class X Chapter 8 โ Trigonometric Identities Maths
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15. If sin cos cos sin ,x a b and y a b prove that 2 2 2 2.x y a b
Sol:
๐บ๐๐ฃ๐๐: ๐ฅ = asin ๐ + ๐๐๐๐ ๐
๐๐๐ข๐๐๐๐๐ ๐๐๐กโ ๐ ๐๐๐๐ , ๐ค๐ ๐๐๐ก:
๐ฅ2 = ๐2 sin2 ๐ + 2๐๐๐ ๐๐๐ cos ๐ + ๐2 cos2 ๐ โฆ (๐)
๐ด๐๐ ๐, ๐ฆ = acos ๐ โ ๐๐ ๐๐ ๐
๐๐๐ข๐๐๐๐๐ ๐๐๐กโ ๐ ๐๐๐๐ , ๐ค๐ ๐๐๐ก:
๐ฆ2 = ๐2 cos2 ๐ โ 2๐๐๐ ๐๐๐ cos ๐ + ๐2 sin2 ๐ โฆ (๐๐)
โด ๐ฟ๐ป๐ = ๐ฅ2 + ๐ฆ2
= ๐2 sin2 +2๐๐๐ ๐๐๐ cos ๐ + ๐2 cos2 ๐ + ๐2 cos2 ๐ โ 2๐๐๐ ๐๐๐ cos ๐ +
๐2 sin2 ๐ [๐ข๐ ๐๐๐ (๐)๐๐๐ (๐๐)]
=๐2(sin2 ๐ + cos2 ๐) + ๐2(sin2 ๐ + cos2 ๐)
=๐2 + ๐2 [โต sin2 ๐ + cos2 ๐ = 1]
=๐ ๐ป๐
๐ป๐๐๐๐ ๐๐๐๐ฃ๐๐.
16. Prove that 21 sin
sec tan .1 sin
Sol: (1+sin ๐)
(1โsin ๐)= (sec ๐ + tan ๐)2
๐ฟ๐ป๐ =(1+sin ๐)
(1โsin ๐)
๐๐ข๐๐ก๐๐๐๐ฆ๐๐๐ ๐กโ๐ ๐๐ข๐๐๐๐๐ก๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐ก๐๐ ๐๐ฆ (1 + sin ๐), ๐ค๐ ๐๐๐ก: (1+sin ๐)2
1โsin2 ๐
=1+2 sin ๐+sin2 ๐
cos2 ๐ [โต sin2 ๐ + cos2 ๐ = 1]
= sec2 ๐ + 2 รsin ๐
cos ๐ร sec ๐ + tan2 ๐
= sec2 ๐ + 2ร tan ๐ ร sec ๐ + tan2 ๐
= (sec ๐ + tan ๐)2
= ๐ ๐ป๐
๐ป๐๐๐๐ ๐๐๐๐ฃ๐๐.
17. Prove that
1 1 1 1.
sec tan sec tancos cos
Sol: 1
(sec ๐โtan ๐)โ
1
cos ๐=
1
cos ๐โ
1
(sec ๐+tan ๐)
๐ฟ๐ป๐ =1
sec ๐โtan ๐โ
1
cos ๐
Class X Chapter 8 โ Trigonometric Identities Maths
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=sec ๐+tan ๐
sec2 ๐โtan2 ๐โ sec ๐
=sec ๐ + tan ๐ โ sec ๐ [โต sec2 ๐ โ tan2 ๐ = 1]
=tan ๐
๐ ๐ป๐ =1
cos ๐โ
1
sec ๐+tan ๐
= sec ๐ โ(sec ๐โtan ๐)
sec2 ๐โtan2 ๐ (๐๐ข๐๐ก๐๐๐ฆ๐๐๐ ๐กโ๐ ๐๐ข๐๐๐๐๐ก๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐ก๐๐ ๐๐ฆ (sec ๐ โ
tan ๐))
=sec ๐ + tan ๐ โ sec ๐ [โต sec2 ๐ โ tan2 ๐ = 1]
=tan ๐
โด๐ฟ๐ป๐ = ๐ ๐ป๐
๐ป๐๐๐๐ ๐๐๐๐ฃ๐๐
18. Prove that
3
3
sin 2sintan .
2cos cos
A AA
A A
Sol:
๐ฟ๐ป๐ =(sin ๐ดโ2 sin3 ๐ด)
(2 cos3 ๐ดโcos ๐ด)
=sin ๐ด(1โ2 sin2 ๐ด)
cos ๐ด(2 cos2 ๐ดโ1)
= tan ๐ด {(sin2 ๐ด+cos2 ๐ดโ2 sin2 ๐ด)
2 cos2 ๐ดโsin2 ๐ดโcos2 ๐ด} [โต sin2 ๐ด + cos2 ๐ด = 1]
= tan ๐ด {(cos2 ๐ดโsin2 ๐ด)
(cos2 ๐ดโsin2 ๐ด)}
= tan ๐ด
= ๐ ๐ป๐
19. Prove that
tan cot
1 tan cot .1 cot 1 tan
A AA A
A A
Sol:
๐ฟ๐ป๐ =tan ๐ด
(1โcot ๐ด)+
cot ๐ด
(1โtan ๐ด)
=tan ๐ด
(1โcot ๐ด)+
cot2 ๐ด
(cot ๐ดโ1) [โต tan ๐ด =
1
cot ๐ด]
=tan ๐ด
(1โcot ๐ด)โ
cot2 ๐ด
(1โcot ๐ด)
=tan ๐ดโcot2 ๐ด
(1โcot ๐ด)
=(
1
cot ๐ด)โcot2 ๐ด
(1โcot ๐ด)
=1โcot3 ๐ด
cot ๐ด(1โcot ๐ด)
=(1โcot ๐ด)(1+cot ๐ด+cot2 ๐ด)
cot ๐ด(1โcot ๐ด) [โต ๐3 โ ๐3 = (๐ โ ๐)(๐2 + ๐๐ + ๐2)]
Class X Chapter 8 โ Trigonometric Identities Maths
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=1
cot ๐ด+
cot2 ๐ด
cot ๐ด+
cot ๐ด
cot ๐ด
= 1 + tan ๐ด + cot ๐ด
= ๐ ๐ป๐
๐ป๐๐๐๐ ๐๐๐๐ฃ๐๐
20. If sec5 cos 36 5A ec A and A is an acute angle , show that 21 .A
Sol:
๐บ๐๐ฃ๐๐: ๐ ๐๐5๐ด = cos ๐๐(๐ด โ 360)
=> ๐๐๐ ๐๐(900 โ 5๐ด) = cos ๐๐(๐ด โ 360) [โต cos ๐๐(900 โ ๐) = sec ๐]
=> 900 โ 5๐ด = ๐ด โ 360
=> 6๐ด = 900 + 360
=> 6๐ด = 1260
=> ๐ด = 210
Multiple Choice Question
1.
sec30?
cos 60ec
(a) 2
3 (b)
3
2
(c) 3 (d) 1
Answer: (d) 1
Sol:
Sec 300
๐๐๐ ๐๐ 600 =sec 300
sec(900โ600)=
sec 300
sec 300 = 1
2. tan 35 cot 78
?cot 55 tan12
(a) 0 (b) 1
(c) 2 (d) none of these
Answer: (c) 2
Sol:
We have,
Tan 350
cot 550+
cot 780
tan 120
= tan 350
cot(900โ350)+
cot(900โ120)
tan 120
= tan 350
tan 350 +tan 120
tan 120 [โต cot(900 โ ๐) = tan ๐]
= 1 + 1 = 2
Class X Chapter 8 โ Trigonometric Identities Maths
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3. tan10 tan15 tan 75 tan 80 =?
(a) 3 (b) 1
3
(c) -1 (d) 1
Answer: (d) 1
Sol:
We have,
tan 100 tan 150 tan 750 tan 800
= tan 100 ร tan 150 ร tan(900 โ 150)ร tan(900 โ 100)
= tan 100ร tan 150ร cot 150ร cot 100 [โต tan(900 โ ๐) = cot ๐]
= 1
4. tan 5 tan 25 tan 30 tan 65 tan85 ?
(a) 3 (b) 1
3
(c) 1 (d) none of these
Answer: (b) 1
3
Sol:
๐๐ โ๐๐ฃ๐:
tan 50 tan 250 tan 300. tan 650 tan 850
= tan 50 tan 250 tan 300 tan(900 โ 250) tan(900 โ 50)
= tan 50 tan 250 ร 1
โ3ร cot 250 cot 50 [โต tan(900 โ ๐) = cot ๐ ๐๐๐ tan 300 =
1
โ3]
= 1
โ3
5. cos1 cos 2 cos3 ..........cos180 ?
(a) -1 (b) 1
(c) 0 (d) 1
2
Answer: (c) 0
Sol:
Cos 10 cos 20 cos 30 โฆ cos 1800
= cos 10 cos 20 cos 30 โฆ cos 900 โฆ cos(180)0
= 0 [โต cos 900 = 0]
6. 2 2
2 2
2sin 63 1 2sin 27
3cos 17 2 3cos 73
=?
Class X Chapter 8 โ Trigonometric Identities Maths
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(a) 3
2 (b)
2
3
(c) 2 (d) 3
Answer: (d) 3
Sol:
๐บ๐๐ฃ๐๐:2 sin2 630+1+2 sin2 270
3 cos2 170โ2+3 cos2 730
= 2(sin2 630+sin2 270)+1
3(cos2 170+cos2 730)โ2
= 2[sin2 630+sin2(900โ630)]+1
3[cos2 170+cos2(900โ170)]โ2
= 2(sin2 630+cos2 630)+1
3(cos2 170+sin2 170)โ2 [โต sin(900 โ ๐) = cos ๐ ๐๐๐ cos(900 โ ๐) = sin ๐]
= 2ร1+1
3ร1โ2 [โต sin2 ๐ + cos2 ๐ = 1]
= 2+1
3โ2
= 3
1= 3
7. sin 47 cos 43 cos 47 sin 43 ?
(a) sin 4 (b) cos 4
(c) 1 (d) 0
Answer: (c) 1
Sol:
We have:
(sin 430 cos 470 + cos 430 sin 470)
= sin 430 cos(900 โ 430) + cos 430 sin(900 โ 430)
= sin 430 sin 430
+ cos 430 cos 430 [โต cos(900 โ ๐) = sin ๐ ๐๐๐ sin(900 โ ๐) = cos ๐]
= sin2 430 + cos2 430
= 1
8. sec70 sin 20 cos 20 cos 70 ?ec
(a) 0 (b) 1
(c) -1 (d) 2
Answer: (d) 2
Sol:
We have:
Sec 700 sin 200 + cos 200 ๐๐๐ ๐๐ 700
= sin 200
cos 700 +cos 200
sin 700
= sin 200
cos(900โ200)+
cos 200
sin(900โ200)
= sin 200
sin 200 +cos 200
cos 200 [โต cos(900 โ ๐) = sin ๐ ๐๐๐ sin(900 โ ๐) = cos ๐]
Class X Chapter 8 โ Trigonometric Identities Maths
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= 1 + 1
= 2
OR
Sec 700 sin 200 + cos 200 ๐๐๐ ๐๐ 700
= ๐๐๐ ๐๐(900 โ 700) sin 200 + cos 200 sec(900 โ 700)
= ๐๐๐ ๐๐ 200 sin 200 + cos 200 sec 200
= 1
sin 200ร sin 200 + cos 200ร
1
cos 200
= 1 + 1
= 2
9. If sin3 cos( 10 ) 3A A and A is acute then ?A
(a) 35 (b) 25
(c) 20 (d) 45
Answer: (b) 25
Sol:
We have:
[sin 3๐ด = cos (๐ด โ 100)]
= > cos(900 โ 3๐ด) = cos(๐ด โ 100) [โต sin ๐ = cos (900 โ ๐)]
= > 900 โ 3๐ด = ๐ด โ 100
= > โ4๐ด = โ100
= > ๐ด =100
4
= > ๐ด = 250
10. If sec4 cos ( 10 ) 4A ec A and A is acute then ?A
(a) 20 (b) 30
(c) 30 (d) 50
Answer: (a) 20
Sol:
We have,
Sec 4๐ด = ๐๐๐ ๐๐(๐ด โ 100)
โน ๐๐๐ ๐๐ (900 โ 4๐ด) = ๐๐๐ ๐๐(๐ด โ 100)
๐ถ๐๐๐๐๐๐๐๐ ๐๐๐กโ ๐ ๐๐๐๐ , ๐ค๐ ๐๐๐ก
900 โ 4๐ด = ๐ด โ 100
โน4๐ด + ๐ด = 900 + 100
โน 5๐ด = 1000
โน ๐ด =1000
5
โด ๐ด = 200
Class X Chapter 8 โ Trigonometric Identities Maths
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11. If A and B are acute angles such that sin A = cos B then (A +B) =?
(a) 45 (b) 60
(c) 90 (d) 180
Answer: (c) 90
12. If cos 0 then sin ?
(a) sin (b) cos
(c) sin 2 (d) cos2
Answer: (d) cos2
Sol:
We have:
cos(๐ผ + ๐ฝ) = 0
=> cos(๐ผ + ๐ฝ) = cos 900
=> ๐ผ + ๐ฝ = 900
=> ๐ผ = 900 โ ๐ฝ โฆ (๐)
Now, sin(๐ผ โ ๐ฝ)
= sin[(900 โ ๐ฝ) โ ๐ฝ] [๐๐ ๐๐๐ (๐)]
= sin(900 โ 2๐ฝ)
= cos 2๐ฝ [โต sin(900 โ ๐) = cos ๐]
13. sin 45 cos 45 ?
(a) 2sin (b) 2cos
(c) 0 (d) 1
Answer: (c) 0
Sol:
We have:
[sin(450 + ๐) โ cos(450 โ ๐)]
=[sin{900 โ (450 โ ๐)} โ cos(450 โ ๐)]
=[cos(450 โ ๐) โ cos(450 โ ๐)] [โต sin(900 โ ๐) = cos ๐]
= 0
14. 2 2sec 10 cot 80 ?
(a) 1 (b) 0
(c) 3
2 (d)
1
2
Answer: (a) 1
Sol:
We have: (sin 790 cos 110 + cos 790 sin 110)
= sin 790 cos(900 โ 790) + cos 790 sin(900 โ 790)
Class X Chapter 8 โ Trigonometric Identities Maths
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= sin 790 sin 790 + cos 790 cos 790[โต cos(900 โ ๐) = sin ๐ ๐๐๐ sin(900 โ ๐) =cos ๐] = sin2 790 + cos2 790
= 1
15. 2 2sec 57 tan 33 ?co
(a)0 (b) 1
(c) -1 (d) 2
Answer: (b) 1
Sol:
We have:
2 2cos 57 tan 33ec
= [๐๐๐ ๐๐2(900 โ 330) โ tan2 330] = (sec2 330 โ tan2 330) [โต cos ๐๐(900 โ ๐) = sec ๐] = 1 [โต sec2 ๐ โ tan2 ๐ = 1]
16. 2 2 2
2 2
2 tan 30 sec 52 sin 38?
cos 70 tan 20ec
(a) 2 (b) 1
2
(c) 2
3 (d)
3
2
Answer: (c) 2
3
Sol:
We have:
[2 tan2 300 sec2 520 sin2 380
cos ๐๐2700โtan2 200 ]
= [2ร(
1
โ3)
2sec2 520{sin2(900โ520)}
{cos ๐๐2(900โ200)}โtan2 200 ]
= [2
3ร
sec2 520.cos2 520
sec2 200โtan2 200] [โต sin(900 โ ๐) = cos ๐ ๐๐๐ ๐๐๐ ๐๐(900 โ ๐) = sec ๐]
= 2
3ร
1
1 [โต sec2 ๐ โ tan2 ๐ = 1]
= 2
3
17.
2 2
2
2 2
sin 22 sin 68sin 63 cos63 sin 27 ?
cos 22 cos 68
(a) 0 (b) 1
(c) 2 (d)3
Class X Chapter 8 โ Trigonometric Identities Maths
______________________________________________________________________________
Answer: (c) 2
Sol:
We have:
[Sin2 220+sin2 680
cos2 220+cos2 68+ sin2 630 + cos 630 sin 270]
= [sin2 220+sin2(900โ220)
cos2(900โ680)+cos2 680 + sin2 630 + cos 630{sin(900 โ 630)}]
= [sin2 220+cos2 220
sin2 680+cos2 680 + sin2 630 + cos 630 cos 630] [โต sin(900 โ ๐))
= cos ๐ ๐๐๐ cos(900 โ ๐) = sin ๐]
= [1
1+ sin2 630 + cos2 630] [โต sin2 ๐ + cos2 ๐ = 1]
= 1 + 1 = 2
18.
2 2cot 90 .sin 90 cot 40
cos 20 cos 70 ?sin tan50
(a) 0 (b)1
(c)-1 (d)none of these
Answer: (b)1
Sol:
We have:
[cot(900โ๐).sin(900โ๐)
sin ๐+
cot 400
tan 500 โ (cos2 200 + cos2 700)]]
=[tan ๐.cos ๐
sin ๐+
cot(900โ500)
tan 500 โ {cos2(900 โ 700) + cos2 700}] [โต cot(900 โ ๐) =
tan ๐ ๐๐๐ sin(900 โ ๐) = cos ๐]
= [sin ๐
sin ๐+
tan 500
tan 500 โ (sin2 700 + cos2 700)] [โต cos(900 โ ๐) = sin ๐]
= (sin ๐
sin ๐+ 1 โ 1)
= 1 + 1 โ 1 = 1
19. cos38 cos 52
?tan18 tan 35 tan 60 tan 72 tan 55
ec
(a) 3 (b) 1
3
(c) 1
3 (d)
2
3
Answer: (c) 1
3
Class X Chapter 8 โ Trigonometric Identities Maths
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Sol:
We have:
[Cos 380 cos ๐๐520
tan 180 tan 350 tan 600 tan 720 tan 550]
=[cos 380 cos ๐๐(900โ380)
tan 180 tan 350รโ3ร tan(900โ180) tan(900โ350)] [โต cos ๐๐(900 โ ๐) = sec ๐ ๐๐๐ tan(900 โ
๐) = cot ๐]
=[cos 380 sec 380
tan 180 tan 350รโ3ร cot 180 cot 350]
=[1
sec 380ร sec 380
1
cot 1801
cot 350รโ3 cot 180 cot 350]
= 1
โ3
20. If 2sin 2 3 then ?
(a) 30 (b) 45
(c) 60 (d) 90
Answer: (a) 30
Sol:
2 sin 2๐ = โ3
โนsin 2๐ =โ3
2= sin 600
โนsin 2๐ = sin 600
โน2๐ = 600
โน๐ = 300
21. If 2cos3 1 then ?
(a) 10 (b) 15
(c) 20 (d) 30
Answer: (c) 20
Sol:
2 cos 3๐ = 1
โน cos 3๐ =1
2
โน cos 3๐ = cos 600 [โต cos 600 = 1
2]
โน 3๐ = 600
โน ๐ =600
3= 200
22. If 3 tan 2 3 0 then ?
(a) 15 (b) 30
Class X Chapter 8 โ Trigonometric Identities Maths
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(c) 45 (d) 60
Answer: (b) 30
Sol:
โ3 tan 2๐ โ 3 = 0
โน โ3 tan 2๐ = 3
โน tan 2๐ = 3
โ3
โน tan 2๐ = โ3 [โต tan 600 = โ3]
โน tan 2๐ = tan 600
โน 2๐ = 600
โน ๐ = 300
23. If tan 3cotx x then x=?
(a) 45 (b) 60
(c) 30 (d) 15
Answer: (b) 60
Sol:
Tan ๐ฅ = 3 cot ๐ฅ
โน tan ๐ฅ
cot ๐ฅ= 3
โน tan2 ๐ฅ = 3 [โต cot ๐ฅ = 1
tan ๐ฅ]
โน tan ๐ฅ = โ3 = tan 600
โน ๐ฅ = 600
24. If tan 45 cos 60 sin 60x then x=?
(a) 1 (b) 1
2
(c) 1
2 (d) 3
Answer: (a) 1
Sol:
๐ฅ tan 450 cos 600 = sin 600 cot 600
โน๐ฅ (1) (1
2) = (
โ3
2) (
1
โ3)
โน ๐ฅ (1
2) = (
1
2)
โน ๐ฅ = 1
25. If 2 2an 45 cos 30 sin 45 cos45t x then x=?
(a) 2 (b) -2
Class X Chapter 8 โ Trigonometric Identities Maths
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(c) 1
2 (d)
1
2
Answer: (c) 1
2
Sol:
(tan2 450 โ cos2 300) = ๐ฅ sin 450 cos 450
โน ๐ฅ =(tan2 450โcos2 300)
sin 450 cos 450
= [(1)2โ(
โ3
2)
2
]
(1
โ2ร
1
โ2)
= (1โ
3
4)
(1
2)
= (
1
4)
(1
2)
= 1
4ร2 =
1
2
26. 2sec 60 1 ?
(a) 2 (b) 3
(c) 4 (d) 0
Answer: (b) 3
Sol:
Sec2 600 โ 1
= (2)2 โ 1
= 4 โ 1
= 3
27. cos0 sin30 sin45 sin90 cos60 cos45 ?
(a) 5
6 (b)
5
8
(c) 3
5 (d)
7
4
Answer: (d) 7
4
Sol:
(cos 00 + sin 300 + sin 450)(sin 900 + cos 600 โ cos 450)
= (1 +1
2+
1
โ2) (1 +
1
2โ
1
โ2)
= (3
2+
1
โ2) (
3
2โ
1
โ2)
= [(3
2)
2
โ (1
โ2)
2
] = (9
4) โ (
1
2) = (
9โ2
4) =
7
4
Class X Chapter 8 โ Trigonometric Identities Maths
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28. 2 2 2sin 30 4cot 45 sec 60 ?
(a) 0 (b) 1
4
(c) 4 (d) 1
Answer: (b) 1
4
Sol:
(Sin2 300 + 4 cot2 450 โ sec2 600)
= [(1
2)
2
+ 4ร(1)2 โ (2)2]
= (1
4+ 4 โ 4) =
1
4
29. 2 2 23cos 60 2cot 30 5sin 45 ?
(a) 13
6 (b)
17
4
(c) 1 (d) 4
Answer: (b) 17
4
Sol:
(3 cos2 600 + 2 cot2 300 โ 5 sin2 450)
= [3ร (1
2)
2
+ 2ร(โ3)2
โ 5ร (1
โ2)
2
]
= [3
4+ 6 โ
5
2]
= 3+24โ10
4=
17
4
30. 2 2 2 2 21
cos 30 cos 45 4sec 60 cos 90 2 tan 60 ?2
(a) 73
8 (b)
75
8
(c) 81
8 (d)
83
8
Answer: (d) 83
8
Sol:
(cos2 300 cos2 450 + 4 sec2 600 +1
2cos2 900 โ 2 tan2 600)
= [(โ3
2)
2
ร (1
โ2)
2
+ 4ร(2)2 +1
2ร(0)2 โ 2ร(โ3)
2]
Class X Chapter 8 โ Trigonometric Identities Maths
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= [(3
4ร
1
2) + 16 โ 6]
= [3
8+ 10]
= 3+80
8=
83
8
31. If cos 10ec then s ?ec
(a) 3
10 (b)
10
3
(c) 1
10 (d)
2
10
Answer: (b) 10
3
Sol:
Let us first draw a right โABC right angled at B and โ ๐ด = ๐.
Given: cosec ๐ = โ10, ๐๐ข๐ก sin ๐ =1
cos ๐๐๐=
1
โ10
Also, sin ๐ =๐๐๐๐๐๐๐๐๐๐ข๐๐๐
๐ป๐ฆ๐๐๐ก๐๐๐ข๐ ๐=
๐ต๐ถ
๐ด๐ถ
So, ๐ต๐ถ
๐ด๐ถ=
1
โ10
Thus, BC = k and AC = โ10๐
Using Pythagoras theorem in triangle ABC, we have:
๐ด๐ถ2 = ๐ด๐ต2 + ๐ต๐ถ2
โน ๐ด๐ต2 = ๐ด๐ถ2 โ ๐ต๐ถ2
โน ๐ด๐ต2 = (โ10๐)2
โ (๐)2
โน ๐ด๐ต2 = 9๐2
โน ๐ด๐ต = 3๐
โด sec ๐ =๐ด๐ถ
๐ด๐ต=
โ10๐
3๐=
โ10
3
Class X Chapter 8 โ Trigonometric Identities Maths
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32. If 8
tan15
then cos ?ec
(a) 17
8 (b)
8
17
(c) 17
15 (d)
15
17
Answer: (a) 17
8
Sol:
Let us first draw a right โABC right angled at B and โ ๐ด = ๐.
Give: tan ๐ =8
5, ๐๐ข๐ก tan ๐ =
๐ต๐ถ
๐ด๐ต
So, ๐ต๐ถ
๐ด๐ต=
8
15
Thus, BC = 8k and AB = 15k
Using Pythagoras theorem in triangle ABC, we have:
๐ด๐ถ2 = ๐ด๐ต2 + ๐ต๐ถ2
โน ๐ด๐ถ2 = (15๐)2 + (8๐)2
โน ๐ด๐ถ2 = 289๐2
โน ๐ด๐ถ = 17๐
โด ๐๐๐ ๐๐ ๐ =๐ด๐ถ
๐ต๐ถ=
17๐
8๐=
17
8
33. If sinb
a then cos ?
(a) 2 2
b
b a (b)
2 2b a
b
(c) 2 2
a
b a (d)
b
a
Answer: (b)
2 2b a
b
Class X Chapter 8 โ Trigonometric Identities Maths
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Sol:
Let us first draw a right โABC right angled at B and โ ๐ด = ๐.
Given: sin ๐ =๐
๐, ๐๐ข๐ก sin ๐ =
๐ต๐ถ
๐ด๐ถ
So, ๐ต๐ถ
๐ด๐ถ=
๐
๐
Thus, BC = ak and AC =bk
Using Pythagoras theorem in triangle ABC, we have:
๐ด๐ถ2 = ๐ด๐ต2 + ๐ต๐ถ2
โน ๐ด๐ต2 = ๐ด๐ถ2 โ ๐ต๐ถ2
โน ๐ด๐ต2 = (๐๐)2 โ (๐๐)2
โน ๐ด๐ต2 = (๐2 โ ๐2)๐2
โน ๐ด๐ต = (โ๐2 โ ๐2)๐
โด cos ๐ =๐ด๐ต
๐ด๐ถ=
โ๐2โ๐2๐
๐๐=
โ๐2โ๐2
๐
34. If tan 3 then sec ?
(a) 2
3 (b)
3
2
(c) 1
2 (d) 2
Answer: (d) 2
Sol:
Let us first draw a right โABC right angled at B and โ ๐ด = ๐.
Give: tan ๐ = โ3
But tan ๐ =๐ต๐ถ
๐ด๐ต
So, ๐ต๐ถ
๐ด๐ต=
โ3
1
Thus, BC = โ3๐ ๐๐๐ ๐ด๐ต = ๐
Class X Chapter 8 โ Trigonometric Identities Maths
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Using Pythagoras theorem, we get:
๐ด๐ถ2 = ๐ด๐ต2 + ๐ต๐ถ2
โน ๐ด๐ถ2 = (โ3๐)2
+ (๐)2
โน ๐ด๐ถ2 = 4๐2
โน ๐ด๐ถ = 2๐
โด sec ๐ =๐ด๐ถ
๐ด๐ต=
2๐
๐=
2
1
35. If 25
sec7
then s ?in
(a) 7
24 (b)
24
7
(c) 24
25 (d) none of these
Answer: (c) 24
25
Sol:
Let us first draw a right โ๐ด๐ต๐ถ right angled at B and โ ๐ด = ๐.
Given sec ๐ =25
7
But cos ๐ =1
๐ ๐๐0=
๐ด๐ต
๐ด๐ถ=
7
25
Thus, ๐ด๐ถ = 25๐ ๐๐๐ ๐ด๐ต = 7๐
Using Pythagoras theorem, we get:
๐ด๐ถ2 = ๐ด๐ต2 + ๐ต๐ถ2
โน ๐ต๐ถ2 = ๐ด๐ถ2 โ ๐ด๐ต2
โน ๐ต๐ถ2 = (25๐)2 โ (7๐)2
โน ๐ต๐ถ2 = 576๐2
Class X Chapter 8 โ Trigonometric Identities Maths
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โน ๐ต๐ถ = 24๐
โด sin ๐ =๐ต๐ถ
๐ด๐ถ=
24๐
25๐=
24
25
36. If 1
sin2
then cot ?
(a) 1
3 (b) 3
(c) 3
2 (d) 1
Answer: (b) 3
Sol:
๐บ๐๐ฃ๐๐: sin ๐ =1
2, ๐๐ข๐ก sin ๐ =
๐ต๐ถ
๐ด๐ถ
๐๐,๐ต๐ถ
๐ด๐ถ=
1
2
Thus, BC = k and AC = 2k
Using Pythagoras theorem in triangle ABC, we have:
๐ด๐ถ2 = ๐ด๐ต2 + ๐ต๐ถ2
๐ด๐ต2 = ๐ด๐ถ2 โ ๐ต๐ถ2
๐ด๐ต2 = (2๐)2 โ (๐)2
๐ด๐ต2 = 3๐2
AB = โ3๐
So, tan ๐ =๐ต๐ถ
๐ด๐ต=
๐
โ3๐=
1
โ3
โด cot ๐ =1
tan ๐= โ3
37. If 4
cos5
then tan =?
(a) 3
4 (b)
4
3
(c) 3
5 (d)
5
3
Answer: (a) 3
4
Class X Chapter 8 โ Trigonometric Identities Maths
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Sol:
Since cos ๐ =4
5 ๐๐ข๐ก cos ๐ =
๐ด๐ต
๐ด๐ถ
So, ๐ด๐ต
๐ด๐ถ=
4
5
Thus, AB = 4k and AC = 5k
Using Pythagoras theorem in triangle ABC, we have:
๐ด๐ถ2 = ๐ด๐ต2 + ๐ต๐ถ2
โน ๐ต๐ถ2 = ๐ด๐ถ2 โ ๐ด๐ต2
โน ๐ต๐ถ2 = (5๐)2 โ (4๐พ)2
โน ๐ต๐ถ2 = 9๐2
โน BC=3k
โด tan ๐ =๐ต๐ถ
๐ด๐ต=
3
4
38. If 3 cosx ec and 3
cotx
then 2
2
1?x
x
(a) 1
27 (b)
1
81
(c) 1
3 (d)
1
9
Answer: (c) 1
3
Sol:
๐บ๐๐ฃ๐๐: 3ร = ๐๐๐ ๐๐ ๐๐๐๐3
๐ฅ= cot ๐
๐ด๐๐ ๐, ๐ค๐ ๐๐๐ ๐๐๐๐ข๐๐ ๐กโ๐๐ก ๐ฅ =cos ๐๐๐
3 ๐๐๐
1
๐ฅ=
cot ๐
3
So, substituting the values of x and 1
๐ฅ ๐๐ ๐กโ๐ ๐๐๐ฃ๐๐ ๐๐ฅ๐๐๐๐ ๐ ๐๐๐, ๐ค๐ ๐๐๐ก:
3 (๐ฅ2 โ1
๐ฅ2) = 3 ((cos ๐๐ ๐
3)
2
โ (cot ๐
3)
2
)
= 3 ((cos ๐๐2๐
9) โ (
cot2 ๐
9))
= 3
9 (cos ๐๐2๐ โ cot2 ๐)
Class X Chapter 8 โ Trigonometric Identities Maths
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= 1
3 [๐ต๐ฆ ๐ข๐ ๐๐๐ ๐กโ๐ ๐๐๐๐๐ก๐๐ก๐ฆ: (cos ๐๐2๐ โ cot2 ๐ = 1)]
39. If 2
2 sec tanx Aand Ax
then 2
2
12 ?x
x
(a) 1
2 (b)
1
4
(c) 1
8 (d)
1
16
Answer: (a) 1
2
Sol:
๐บ๐๐ฃ๐๐: 2๐ฅ = sec ๐ด ๐๐๐2
๐ฅ= tan ๐ด
๐ด๐๐ ๐, ๐ค๐ ๐๐๐ ๐๐๐๐ข๐๐ ๐กโ๐๐ก ๐ฅ =sec ๐ด
2 ๐๐๐
1
๐ฅ=
tan ๐ด
2
So, substituting the values of x and 1
๐ฅ ๐๐ ๐กโ๐ ๐๐๐ฃ๐๐ ๐๐ฅ๐๐๐๐ ๐ ๐๐๐, ๐ค๐ ๐๐๐ก:
2 (๐ฅ2 โ1
๐ฅ2) = 2 ((sec ๐ด
2)
2
โ (tan ๐ด
2)
2
)
= 2 ((sec2 ๐ด
4) โ (
tan2 ๐ด
4))
= 2
4 (sec2 ๐ด โ tan2 ๐ด)
= 1
2 [๐ต๐ฆ ๐ข๐ ๐๐๐ ๐กโ๐ ๐๐๐๐๐ก๐๐ก๐ฆ: (sec2 ๐ โ tan2 ๐ = 1) ]
40. If 4
tan3
then sin cos ?
(a) 7
3 (b)
7
4
(c) 7
5 (d)
5
7
Answer: (c) 7
5
Sol:
Let us first draw a right โABC right angled at B and โ ๐ด = ๐.
Class X Chapter 8 โ Trigonometric Identities Maths
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Tan ๐ =4
3=
๐ต๐ถ
๐ด๐ต
So, AB = 3k and BC = 4k
Using Pythagoras theorem, we get:
๐ด๐ถ2 = ๐ด๐ต2 + ๐ต๐ถ2
โน ๐ด๐ถ2 = (3๐)2 + (4๐)2
โน ๐ด๐ถ2 = 25๐2
โน ๐ด๐ถ = 5๐
Thus, sin ๐ =๐ต๐ถ
๐ด๐ถ=
4
5
And cos ๐ =๐ด๐ต
๐ด๐ถ=
3
5
โด (sin ๐ + cos ๐) = (4
5+
3
5) =
7
5
41. If tan cot 5 then 2 2tan cot ?
(a) 27 (b) 25
(c) 24 (d) 23
Answer: (d) 23
Sol:
We have (tan ๐ + cot ๐) = 5
Squaring both sides, we get:
(Tan ๐ + cot ๐)2 = 52
โน tan2 ๐ + cot2 ๐ + 2 tan ๐ cot ๐ = 25
โน tan2 ๐ + cot2 ๐ + 2 = 25 [โต tan ๐ =1
cot ๐]
โน tan2 ๐ + cot2 ๐ = 25 โ 2 = 23
42. If 5
cos sec2
then 2 2cos sec ?
(a) 21
4 (b)
17
4
(c) 29
4 (d)
33
4
Class X Chapter 8 โ Trigonometric Identities Maths
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Answer: (b) 17
4
Sol:
We have (cos ๐ + sec ๐) =5
2
Squaring both sides, we get:
(Cos ๐ + sec ๐)2 = (5
2)
2
โน cos2 ๐ + sec2 ๐ + 2๐ =25
4
โน cos2 ๐ + sec2 ๐ + 2 =25
4 [โต sec ๐ =
1
cos ๐]
โน cos2 ๐ + sec2 ๐ =25
4โ 2 =
17
4
43. If 1
tan7
then
2 2
2 2
cos sec
cos sec
ec
ec
=?
(a)2
3
(b)
3
4
(c) 2
3 (d)
3
4
Answer: (d) 3
4
Sol:
= ๐๐๐ ๐๐2๐โsec2 ๐
๐๐๐ ๐๐2๐+sec2 ๐
= sin2 ๐(
1
sin2 ๐โ
1
cos2 ๐)
sin2 ๐(1
sin2 ๐+
1
cos2 ๐) [๐๐ข๐๐ก๐๐๐ฆ๐๐๐ ๐กโ๐ ๐๐ข๐๐๐๐๐ก๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐ก๐๐ ๐๐ฆ sin2 ๐]
= 1โtan2 ๐
1+tan2 ๐
= 1โ
1
7
1+1
7
=6
8=
3
4
44. If 7 tan 4 then
7sin 3cos?
7sin 3cos
(a) 1
7 (b)
5
7
(c) 3
7 (d)
5
14
Answer: (a) 1
7
Sol:
Class X Chapter 8 โ Trigonometric Identities Maths
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7 tan ๐ = 4
๐๐๐ค, ๐๐๐ฃ๐๐๐๐๐ ๐กโ๐ ๐๐ข๐๐๐๐๐ก๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐ก๐๐ ๐๐ ๐กโ๐ ๐๐๐ฃ๐๐ ๐๐ฅ๐๐๐๐ ๐ ๐๐๐ ๐๐ฆ cos ๐,
We get: 1
cos ๐(7 sin ๐โ3 cos ๐)
1
cos ๐ (7 sin ๐+3 cos ๐)
= 7 tan ๐โ3
7 tan ๐+3
= 4โ3
4+3 [โต 7 tan ๐ = 4]
= 1
7
45. If 3cot 4 then
5sin 3cos?
5sin 3cos
(a) 1
3 (b) 3
(c) 1
9 (d) 9
Answer: (d) 9
Sol:
๐๐ โ๐๐ฃ๐(5 sin ๐+3 cos ๐)
(5 sin ๐โ3 cos ๐)
๐ท๐๐ฃ๐๐๐๐๐ ๐กโ๐ ๐๐ข๐๐๐๐๐ก๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐ก๐๐ ๐๐ ๐กโ๐ ๐๐๐ฃ๐๐ ๐๐ฅ๐๐๐๐ ๐ ๐๐๐ ๐๐ฆ sin ๐, ๐ค๐ ๐๐๐ก: 1
sin ๐(5 sin ๐+3 cos ๐)
1
sin ๐(5 sin ๐โ3 cos ๐)
= 5+3 cot ๐
5โ3 cot ๐
= 5+4
5โ4= 9 [โต 3 cot ๐ = 4]
46. If tana
b then
sin cos?
sin cos
a b
a b
(a)
2 2
2 2
a b
a b
(b)
2 2
2 2
a b
a b
(c)
2
2 2
a
a b (d)
2
2 2
a
a b
Answer: (b)
2 2
2 2
a b
a b
Sol:
Class X Chapter 8 โ Trigonometric Identities Maths
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๐๐ โ๐๐ฃ๐ tan ๐ =๐
๐
๐๐๐ค, ๐๐๐ฃ๐๐๐๐๐ ๐กโ๐ ๐๐ข๐๐๐๐๐ก๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐ก๐๐ ๐๐ ๐กโ๐ ๐๐๐ฃ๐๐ ๐๐ฅ๐๐๐๐ ๐ ๐๐๐ ๐๐ฆ cos ๐
We get: ๐ sin ๐โ๐ cos ๐
๐ sin ๐+๐ cos ๐
=
1
cos ๐(๐ sin ๐โ๐ cos ๐)
1
cos ๐(๐ sin ๐+๐ cos ๐)
=๐ tan ๐โ๐
๐ tan ๐+๐=
๐2
๐โ๐
๐2
๐+๐
=๐2โ๐2
๐2+๐2
47. If 2sin sin 1A A then 2 4cos cos ?A A
(a) 1
2 (b) 1
(c) 2 (d) 3
Answer: (b) 1
Sol:
Sin ๐ด + sin2 ๐ด = 1
= > sin ๐ด = 1 โ sin2 ๐ด
= > sin ๐ด = cos2 ๐ด (โต 1 โ sin2 ๐ด)
= > sin2 ๐ด = cos4 ๐ด (๐๐๐ข๐๐๐๐๐ ๐๐๐กโ ๐ ๐๐๐๐ )
= > 1 โ cos2 ๐ด = cos4 ๐ด
= > cos4 ๐ด + cos2 ๐ด = 1
48. If 2cos cos 1A A then 2 4sin sin ?A A
(a) 1 (b) 2
(c) 4 (d) 3
Answer: (a) 1
Sol:
cos ๐ด + cos2 ๐ด = 1
=> cos ๐ด = 1 โ cos2 ๐ด
=> cos ๐ด = sin2 ๐ด (โต 1 โ cos2 ๐ด = ๐ ๐๐2)
=> cos2 ๐ด = sin4 ๐ด (๐ด๐๐ข๐๐๐๐๐ ๐๐๐กโ ๐ ๐๐๐๐ )
=> 1 โ sin2 ๐ด = sin4 ๐ด
=> sin4 ๐ด + sin2 ๐ด = 1
49. 1 sin
?1 sin
A
A
(a) sec tanA A (b) ec tans A A
(c) sec tanA A (d) none of these
Answer: (b) ec tans A A
Sol:
Class X Chapter 8 โ Trigonometric Identities Maths
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โ1โsin ๐ด
1+sin ๐ด
=โ(1โsin ๐ด)
(1+sin ๐ด)ร
(1โsin ๐ด)
(1โsin ๐ด) [๐๐ข๐๐ก๐๐๐๐ฆ๐๐๐ ๐กโ๐ ๐๐๐๐๐๐๐๐๐ก๐๐ ๐๐๐ ๐๐ข๐๐๐๐๐ก๐๐ ๐๐ฆ (1 โ sin ๐ด)]
=(1โsin ๐ด)
โ1โsin2 ๐ด
=(1+sin ๐ด)
โcos2 ๐ด
=(1โsin ๐ด)
cos ๐ด
=1
cos ๐ดโ
sin ๐ด
cos ๐ด
=sec ๐ด โ tan ๐ด
50. 1 cos
?1 cos
A
A
(a) cossec cotA A (b) cossec cotA A
(c) cosec cotA A (d) none of these
Answer: (b) cossec cotA A
Sol:
โ1โcos ๐ด
1+cos ๐ด
=โ(1โcos ๐ด)
(1+cos ๐ด)ร
(1โcos ๐ด)
(1โcos ๐ด) [๐๐ข๐๐ก๐๐๐๐ฆ๐๐๐ ๐กโ๐ ๐๐ข๐๐๐๐๐ก๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐ก๐๐ ๐๐ฆ (1 โ cos ๐ด)]
=โ(1โcos ๐ด)(1โcos ๐ด)
1โcos2 ๐ด
=1โcos ๐ด
โsin2 ๐ด
=1โcos ๐ด
sin ๐ด
=1
sin ๐ดโ
cos ๐ด
sin ๐ด
=cos ๐๐๐ด โ cot ๐ด
51. If tana
b then
cos sin?
cos sin
(a) a b
a b
(b)
a b
a b
(c) b a
b a
(d)
b a
b a
Answer: (c) b a
b a
Sol:
๐บ๐๐ฃ๐๐: tan ๐ =๐
๐
Class X Chapter 8 โ Trigonometric Identities Maths
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๐๐๐ค,(cos ๐+sin ๐)
(cos ๐โsin ๐)
=(1+tan ๐)
(1โtan ๐) [๐ท๐๐ฃ๐๐๐๐๐ ๐กโ๐ ๐๐ข๐๐๐๐๐ก๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐ก๐๐ ๐๐ฆ cos ๐]
=(1+
๐
๐)
(1โ๐
๐)
=(
๐+๐
๐)
(๐โ๐
๐)
=(๐+๐)
(๐โ๐)
52. 2
cos cot ?ec
(a) 1 cos
1 cos
(b)
1 cos
1 cos
(c) 1 sin
1 sin
(d) none of these
Answer: (b) 1 cos
1 cos
Sol: (๐๐๐ ๐๐ ๐ โ cot ๐)2
= (1
sin ๐โ
cos ๐
sin ๐)
2
= (1โcos ๐
sin ๐)
2
=(1โcos ๐)2
(1โcos2 ๐)
=(1โcos ๐)2
(1+cos ๐)(1โcos ๐)
=(1โcos ๐)
(1+cos ๐)
53. sec tan 1 sin ?A A A
(a) sin A (b) cos A
(c) sec A (d) cos ecA
Answer: (b) cos A
Sol:
(sec ๐ด + tan ๐ด)(1 โ sin ๐ด)
=(1
cos ๐ด+
sin ๐ด
cos ๐ด) (1 โ sin ๐ด)
=(1+sin ๐ด
cos ๐ด) (1 โ sin ๐ด)
=(1โsin2 ๐ด
cos ๐ด)
=(cos2 ๐ด
cos ๐ด)
=cos ๐ด