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### Transcript of Chapter 1 Linear Functions and Mathematical Modeling Section 1.2

• Chapter 1 Linear Functions and Mathematical Modeling

Section 1.2

• Section 1.2Slope of a Line, Vertical Intercept, and Rate of Change Concept of Slope

Slope and the Graph of a Line

Slope-Intercept Form of a Line

Slope as Rate of Change

• Slope of a LineMeasure of the steepness of a line

Note: Delta (uppercase ) is the fourth letter of the Greek alphabet, and it is used frequently to represent change. y means "change in y" and x means change in x."

• Find the slope of the line through the points (5, 16) and (7, 8).

or

• Slopes of Lines (Moving from left to right on the graph)

• Slope-Intercept Form of a Line: y = mx + b where m is the slope and b is the y-intercept

Example: y = 3x 18

m b

The slope, m = 3.

The y-intercept, b = 18.

Recall that we can find the y-intercept by letting x = 0 and solving for y. Thus, we can verify that b is the y-intercept: y = 3(0) 18 = 18. So, the y-intercept is (0,18).

Caution: Before identifying m and b, y must be isolated!

• True of False: The line 5y = 20x + 30 is written in slope-intercept form.

Slope-intercept form of a line: y = mx + b Answer: False (y is not isolated)

Changing to slope-intercept form:

5y = 20x + 30 5 5

y = 4x + 6 where m = 4 (slope) and b = 6 (y-intercept)

• Graph the equation by using the slope-intercept method.

Identify the y-intercept and the slope.y-intercept is 2 or (0, 2)slope is 3/4 Plot the y-intercept, (0, 2).From the y-intercept, rise 3 units and run 4 units to find a second point. Connect the points with a straight line.

• Sketch the graph of the line with slope m = 3/5 that passes through the point (5, 5).

Start by plotting the given point, (5, 5).

Slope is 3/5: rise of 3 and run of 5.

From (5, 5), move down 3 units and right 5 units to find a second point.

Connect the points with a straight line.

• Slope as Rate of Change

The rate of change describes how one quantity changes in relation to another quantity.

In the context of linear applications, slope is identified as a constant rate of change.

• Slope as Rate of ChangeA helpful way of interpreting slope in the context of a problem is to identify the units on the slope. Write a ratio where the numerator is what the y represents, and the denominator is what the x represents.

Example: A traveler has \$4500 in his vacations account and plans to spend \$300 per week. The linear equation y = 4500 300x represents the amount of money in the account, y, in terms of the number of weeks, x. Interpret the slope.

Slope:

So, we can writeThe slope is 300 (equivalent to 300/1); this means that the money decreases at a rate of 300 dollars per week.

• A jewelry crafter charges a design fee of \$25 for fine earrings and sells each pair for \$169. The total cost for the earrings, y, can be represented by the equation y = 169x + 25, where x represents the number of pairs of earrings.

a.Find the slope (rate of change) of the total cost equation, and explain its meaning in the context of this problem. The line is given in slope-intercept form, where the slope m = 169.

Excluding the design fee, each pair of earrings costs \$169.

b. Find the vertical intercept (y-intercept)and explain its meaning.The vertical intercept is (0, 25).There is an initial design fee of \$25.

• The graph below shows the amount of gasoline, g, left in the tank fuel of a 5-gallon portable generator after running for h hours.

a.Find the slope, to the nearest tenth, and explain its meaning.b. Write the equation of this line in slope-intercept form.

a. Using the intercepts, (0, 5) and (8, 0):

The fuel is decreasing at a rate of 0.6 gallons per hour.

b. The slope is 0.6 and the vertical intercept is (0, 5), therefore the equation is g = 0.6h + 5 or g = 5 0.6h.

• Using your textbook, practice the problems assigned by your instructor to review the concepts from Section 1.2.

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