Big Ideas Math Integrated I, 2016
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Big Ideas Math Integrated I, 2016 View details
3. Writing Equations of Parallel and Perpendicular Lines
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Exercise 20 Page 181

What similarities and differences do perpendicular lines have?

y=-3/4x-4

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 a given slope and the slope of a line perpendicular to it will be -1. m_1* m_2=-1 For any equation written in slope-intercept form, y= mx+ b, we can identify its slope as the value of m. y= 4/3x+ 6 In the given equation we can see that its slope is 43. By substituting this value into our negative reciprocal equation for m_1, we can solve for the slope of the perpendicular line m_2.

m_1 * m_2 = - 1
4/3* m_2 = - 1
â–¼
Solve for m_2
4m_2=-3
m_2=-3/4
m_2 = -3/4

With this, we can identify that any line perpendicular to the given equation will have a slope of - 34.

Writing the Equation of the Perpendicular Line

With the slope m_2= - 34, we can write a general equation in slope-intercept form for all lines perpendicular to the given equation. y= - 34x+ b By substituting the given point ( -4, -1) into this equation for x and y, we can solve for the y-intercept b of the perpendicular line.

y=-3/4x+b
-1=-3/4( -4)+b
â–¼
Solve for b
-1=3/-4(-4)+b
-1=3+ b
-4=b
b=-4

Now that we have the y-intercept, we can write the equation for the line that is both perpendicular to y= 43x+6 and passes through the point (-4,-1). y= -3/4x -4