Big Ideas Math Integrated I, 2016
BI
Big Ideas Math Integrated I, 2016 View details
3. Writing Equations of Parallel and Perpendicular Lines
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Exercise 24 Page 181

Practice makes perfect
a

Parallel lines have exactly the same slope. Therefore, to find the slope of a parallel line, we need to know the slope of the given line. We can calculate it by substituting points that lie on the given line into the Slope Formula. Let's use (1,- 4) and (2,- 1).

m=y_2-y_1/x_2-x_1
m=-1-( -4)/2- 1
â–¼
Simplify right-hand side
m=-1+4/2-1
m=3/1
m=3
The slope of the given line — and, consequently, of the parallel line — is 3. Next, by substituting the slope and the given point, ( 3, -2), into the slope-intercept form, we can find the y-intercept of the desired line.

y=mx+b
-2= 3( 3)+b
â–¼
Solve for b
-2=9+b
-11=b
b=-11

We can now use the slope and y-intercept to write the equation of the parallel line in slope-intercept form. y=3x-11

b

We want to find the equation of a perpendicular line through the given point. When two lines are perpendicular, their slopes are negative reciprocals. This means that the product of their slopes must be -1.

m_1* m_2=- 1 From Part A, we know that the slope of the given line is m=3. We can substitute this into the above equation to solve for the slope of the perpendicular line.

m_1* m_2=- 1
3* m_2=- 1
m_2=-1/3
m_2=-1/3

The slope of the perpendicular line is - 13. Once again, by substituting the slope and the given point, ( 3, -2), into the slope-intercept form, we can find the y-intercept.

y=mx+b
-2= -1/3( 3)+b
â–¼
Solve for b
-2=-1+b
-1=b
b=-1

We can now use the slope and y-intercept to write the equation of the perpendicular line through (3,- 2). y=-1/3x-1