Big Ideas Math: Modeling Real Life, Grade 8
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Big Ideas Math: Modeling Real Life, Grade 8 View details
3. Linear Functions
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Exercise 7 Page 293

Practice makes perfect

Consider the given figure.

We are given a table that shows a familiar pattern from geometry.

x 1 2 3 4 5
y π
From the given figure, we can see that x represents the diameter of the given circle. To find the meaning of y, recall that functions written in slope-intercept form follow a specific format. y= mx+ b In this form, m is the slope and b is the y-intercept. We need to identify these values using the given table. Let's start by substituting the points (1,π) and (2,2π), into the slope formula and calculating m.
m = y_2-y_1/x_2-x_1
m=2π- π/2- 1
Simplify right-hand side
m=π/1
m=π
We calculated that the slope of the given equation is π. Now, remember that the y-intercept is the y-value where the line crosses the y-axis. It occurs when x=0. Since the slope is equal to π, we need to subtract 1 unit to x-coordinate and π units to y-coordinate of the first given point to obtain the previous point.
(x_1,y_1) (x-1,y-2) (x_2,y_2)
(1,π) (1- 1,π- π) (0,0)
Notice that we obtain a point where x=0. This means that the function intercepts the y-axis at (0, 0). Therefore, the value of b is 0. Now that we have the slope and the y-intercept, we can write a function for the line that passes through the given points. y= πx+ 0 → y=π x Now, recall the formula for the circumference of a circle. C= π d Here, d represents the diameter of the circle and C the circumference. In our case, the diameter is equal to x. Let's substitute this value into the formula.
C= π d
C= π * x
C= xπ
We get the same form as the function obtained with the given data. Therefore, x represents the diameter and y the circumference of the circle.
Remember that a linear function can be written in the following form.

y= mx+ b Let's compare this function with the obtained in Part A. y=π x → y= πx+ 0 This means that our function is a linear function.