Big Ideas Math Integrated I, 2016
BI
Big Ideas Math Integrated I, 2016 View details
3. Writing Equations of Parallel and Perpendicular Lines
Continue to next subchapter

Exercise 19 Page 181

What similarities and differences do perpendicular lines have?

y=-2x+24

Practice makes perfect

To write the equation of a line perpendicular to the given equation, we first need to determine its slope. After that, we will write a general equation and use the given point to determine the y-intercept.

Calculating the Slope of the Perpendicular Line

Two lines are perpendicular when their slopes are negative reciprocals. This means that the product of the slopes of perpendicular lines is -1. m_1* m_2=-1 Note that the given equation is written in slope-intercept form. y= 1/2x-9 In the above formula we can see that the slope is 12. By substituting this value for m_1 in our negative reciprocal equation, we can solve for the slope of the perpendicular line m_2.

m_1 * m_2 = - 1
1/2* m_2 = - 1
m_2 = -2

Any perpendicular line to the given equation will have a slope of -2.

Writing the Equation of the Perpendicular Line

Let's write a partial equation in slope-intercept form for a line perpendicular to the given equation. y= -2x+ b By substituting the given point ( 7, 10) into this equation for x and y, respectively, we can solve for the y-intercept b of the perpendicular line.

y=-2x+b
10=-2( 7)+b
â–¼
Solve for b
10=-14 + b
24=b
b=24

Now that we have the y-intercept, we can write the equation of the perpendicular line to y= 12x-9 through the point (7,10). y= -2x+ 24