Big Ideas Math: Modeling Real Life, Grade 8
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6. Writing Equations in Slope-Intercept Form
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Exercise 14 Page 177

First, use the given points to calculate the slope. Then, identify the y-intercept.

y=-1/2x+1

Practice makes perfect
An equation in slope-intercept form follows a specific format. y= mx+ b For an equation in this form, m is the slope and b is the y-intercept. We will start by looking at the given table.
x y
- 2 2
0 1
2 0
4 - 1
Let's now substitute the first two points given in the table, (- 2,2) and (0,1), into the Slope Formula and calculate m.
m = y_2-y_1/x_2-x_1
m=1- 2/0-( - 2)
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Simplify right-hand side
m=- 1/0-(- 2)
m=- 1/0+2
m=- 1/2
m=- 1/2
We calculated that the slope of the given equation is - 12, which means that for every 2 horizontal steps in the positive direction, we take 1 vertical step in the negative direction. Now that we know the slope, we can write a partial version of the equation. y=- 1/2 x+ b To complete the equation, we also need to determine the y-intercept b. We can consider the given table and find the value of y when x=0.
x y
- 2 2
0 1
2 0
4 - 1

Since y= 1 when x=0, the y-intercept is 1. This means that the line crosses the y-axis at the point (0, 1). We can now complete the equation. y= -1/2x+ 1