Big Ideas Math: Modeling Real Life, Grade 8
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7. Writing Equations in Point-Slope Form
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Exercise 18 Page 183

Point-slope form is y-y_1=m(x-x_1).

y=-1/3x+2

Practice makes perfect
Equations in point-slope form follow a specific format. y- y_1= m(x- x_1) In this form, m is the slope of the line and ( x_1, y_1) is a point on the line. Here, we are given that the line passes through two known points. (-9,5) and (-3,3) To determine the slope of the line, we use the Slope Formula.
m=y_2-y_1/x_2-x_1
m=3- 5/-3-( -9)
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Simplify right-hand side
m=3-5/-3+9
m=-2/6
m=-2/6
m=-2/2/6/2
m= -1/3
Now that we know the slope of the line is - 13, we can write the equation of the line in point-slope form. We can use either of the given points as (x_1,y_1) in our equation. Let's use ( -9, 5).
y-y_1=m(x-x_1)
y- 5= -1/3(x-( -9))
y-5=-1/3(x+9)
Since any point on the line could be used to form a point-slope equation, there are infinitely many possible equations. However, we are asked to write the equation in slope-intercept form, so this will give us a unique equation for this line.
y-5=-1/3(x+9)
y-5=(-1/3)x+(-1/3)9
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Simplify
y-5=(-1/3)x+(-1/3)9
y-5=-1 * x/3+(-1 * 9/3)
y-5=- x/3+(-9/3)
y-5=-x/3+(-9/3)
y-5=-x/3+(-3)
y-5=-x/3-3
y-5+5=-x/3-3+5
y=-x/3+2
y=-1/3x+2