Circumference of a Circle - cvut.cz

Circumference of a Circle

The line segment AB, |AB| = 2r , and its interior point X are given. Thesum of the lengths of semicircles over the diameters AX and XB is

3πr ; πr ;3

2πr ;

5

4πr ;

1

2πr ;

Sarka Voracova Plane Geometry May 30, 2018 1 / 26

Circumscribed circle

Circumcircle of a polygon is a circle which passes through all the verticesof the polygon.

Inscribed circle

Incircle of a polygon is a circle touches (is tangent to) all sides of thepolygon.


Circle Inscribed in Hexagon

The ratio of the area of a regular hexagon with side a to the area of thecircle inscribed in it is

2√

3 : π,√

3 : π, 2√

2 : π, 3√

2 : π,√

2 : π,


Inscribed triangle

Consider an equilateral triangle ABC with the side length a and aninscribed equilateral triangle DEF , where D ∈ AB, E ∈ BC , F ∈ CAand the area of the triangle DEF is equal to one third of the area of thetriangle ABC . The length of the side of the triangle DEF is

a√2

;a

2;

a

4;

a√6

;a√3

;


Consider an equilateral triangle ABC with the side length a and aninscribed equilateral triangle DEF , where D ∈ AB, E ∈ BC , F ∈ CAand the area of the triangle DEF is equal to one half of the area of thetriangle ABC . The length of the side of the triangle DEF is

a

2;

a√3

;a

4;

a√2

;a√6

;


Cartesian Coordinate System in the Plane

Any point P in the plane can be located byunique ordered pair on numbers –coordinates. We write P[x , y ] or P = (x , y).

Example

Describe and sketch the regions given by following sets

{(x , y); x ≥ 0} {(x , y); y = 1} {(x , y); y < 1}


Square in CCS

Determine coordinates of vetices C ,D of the square ABCD, whereA[1,−1],B[5, 1]1

1squareSarka Voracova Plane Geometry May 30, 2018 7 / 26

Lines – Slope form of the equation

Slope m – measure of the steepness. Theslope of line AB is

m =∆y

∆x.

Let line l passes through a given pointA = (xA, yA) and has slope m.

X = (x , y) ∈ l ⇔ y − yAx − xA

= m

y − yA = m(x − xA)

y = mx − xA + yA


Lines – Slope and general equation

Example

Write the equation of the line l = AB, where A = (2, 1) and B = (0, 3).

Determine:

The slope m of the line l .

Intersection points with coordinate axes, P = l ∩ x , Q = l ∩ y .

whether the line l goes through the point P = (3, 4).


Identical Straight Lines2

Straight lines 6x + by + c = 0 and AB, where A[1, 2], B[2, −1], areidentical only if

b = 4 ∧ c = 10

b = 4 ∧ c = −14

b = 2 ∧ c = −10

b = −4 ∧ c = 2

b = −2 ∧ c = 10

2IdenticalLines.ggbSarka Voracova Plane Geometry May 30, 2018 10 / 26

Lines – Parametric equations

Directional vector of line AB is any vectorparallel with ~u = B − A. Directional vector~u is perpendicular to normal vector ~n, i.e.~u · ~n = 0.Let line l passes through a given pointA = (xA, yA) and has directional vector ~u.

X = (x , y) ∈ l ⇔ ~AX = t · ~uX − A = t · ~u

X = A + t · ~u


Point of Intersection3

A straight line x + 2y − 7 = 0 and a line segmentx = 1 + 4t, y = 1 + 2t, t ∈ 〈0, 1〉,

do not intersect

intersect at point [5, 1]



intersect at point [4, 32 ]

3IntersectionPoint.ggbSarka Voracova Plane Geometry May 30, 2018 12 / 26

Perpendicular Bisector4

A straight line ax − 2y + c = 0 is the axis of the line segment AB, whereA[1, 5], B[−3, 3], only if

a = −4 ∧ c = 4

a = 1 ∧ c = 3

a = 4 ∧ c = −4

a = 2 ∧ c = −2

a = 7 ∧ c = 0

4PerpendicularBisectorSarka Voracova Plane Geometry May 30, 2018 13 / 26

Reflection in Line5

Given a line a and a point P. Reflection P ′ of P in a is the point such thatPP ′ is perpendicular to a, and PM = MP ′, where M is the point ofintersection of PP ′ and a. In other words, P ′ is located on the other side ofaxis, but at the same distance from a as P. P ′ is said to be a mirror of P.

5ReflectionLineChB.ggbSarka Voracova Plane Geometry May 30, 2018 14 / 26

Reflection in Line6

Axial symmetry with an axis p : x − y = 0 maps a point A[3, 0] toa point A[?, ?]

6ReflectionLine.ggbSarka Voracova Plane Geometry May 30, 2018 15 / 26

Reflection in Line

Axial symmetry with an axis p : x − 2y + 1 = 0 maps a point A[4, 0]to a point

[0, 4]

[0, 3]

[3, 2]

[2, −2]

[2, 4]


Altitude of a Triangle7

A straight line containing the height ha of the triangle ABC , whereA[0, −1], B[6, 0], C [4, 3], has an equation

7AltitudeTriangle.ggbSarka Voracova Plane Geometry May 30, 2018 17 / 26

Altitude of a Triangle

A straight line containing the height hc of the triangle ABC , whereA[−1, 1], B[3, 2], C [2, 5], has an equation

4x − y − 3 = 0

x + 4y − 22 = 0

x − 4y + 18 = 0

x + 4y + 13 = 0

4x + y − 13 = 0


Equation of the Circle

Circle is the set of all points X in a plane that are at a given distance rfrom a given point, the centre O.X [x , y ]; O[m, n]

r = |OX | = |(X − O)| =√

(x −m)2 + (x − n)2

r2 = (x −m)2 + (y − n)2


Circumcircle and Incircle8

Write the equation for circle inscribed(circumscribed) in a square ABCD,where A[1,−1], B[2, 2]

8InscribedSquare.ggbSarka Voracova Plane Geometry May 30, 2018 20 / 26

Circumcircle about Rectangle

The equation of a circle circumscribed about the rectangle ABCD, whereA[2, −3], C [8, 3], is

(x − 3)2 + (y − 3)2 = 36

x2 + 10x + y2 − 18 = 0

x2 − 10x + y2 + 7 = 0

(x − 10)2 + y2 − 72 = 0

x2 − 10x + y2 + 28 = 0


Inscribed Circle

The equation of a circle inscribed in the square ABCD, where A[2, 1],C [4, 11], is

x2 + y2 − 6x − 12y + 32 = 0

x2 + y2 − 3x − 6y − 29 = 0

x2 + y2 + 6x + 12y − 32 = 0

x2 + y2 + 6x − 12y + 32 = 0

x2 + y2 − 6x + 12y + 29 = 0


Circumcircle about Triangle

Write the equation of the circle c circumscribed about the triangle ABC ,where A[0, 0], B[2, 0], C [0, 2]


The equation of a circle circumscribed about the triangle ABC , whereA[1, 5], B[9, 1], C [1, 1], is

x2 + y2 − 10x − 6y + 14 = 0

x2 + y2 − 5x − 3y + 20 = 0

x2 + y2 − 10x + 6y − 20 = 0

x2 + y2 + 10x + 6y − 54 = 0

x2 + y2 + 5x + 3y + 20 = 0


Nearest and Furthest point

Consider a circle given by the equation x2 + y2 − 16x − 12y + 75 = 0 .The ratio of distances of the nearest and furthest points of this circle fromthe origin of the coordinate system is

1 : 11 1 : 3 2 : 3 1 : 2 3 : 13


Circle and Tangent Line9

Consider a circle with the center S [−1, 3] and a tangent t given by theequation x − 2y + 2 = 0 . The equation of this circle is

x2 + y2 + 2x − 6y + 5 = 0

9CircleTangent.ggbSarka Voracova Plane Geometry May 30, 2018 26 / 26

Circumference of a Circle - cvut.cz

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