McGraw Hill Glencoe Algebra 1, 2012
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McGraw Hill Glencoe Algebra 1, 2012 View details
4. Parallel and Perpendicular Lines
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Exercise 31 Page 243

What similarities and differences do perpendicular lines have?

y=-1/5x-3/25

Practice makes perfect

To write the equation of a line perpendicular to the given equation, we first need to determine its slope.

Calculating the Perpendicular Line's Slope

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. Looking at the given equation, we can see that its slope is 5. y=5x+ 3 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
5* m_2 = - 1
â–Ľ
Solve for m_2
m_2=- 1/5
m_2 = -1/5
With this, we can identify that any line perpendicular to the given equation will have a slope of - 15.

Calculating the Point

Before we continue writing the equation for the perpendicular line, we will calculate the x-intercept of the given equation. To do this, we will substitute 0 for y and solve for x.
y=5x+3
0=5x+3
â–Ľ
Solve for x
- 3=5x
- 3/5=x
-3/5=x
x=-3/5
An x-intercept of - 35 means that the graph passes the x-axis at the point (- 35,0).

Writing the Perpendicular Line's Equation

Using the slope m_2=- 15, we can write a general equation in slope-intercept form for all lines perpendicular to the given equation. y=-1/5x+b By substituting the given point ( - 35, 0) into this equation for x and y, we can solve for the y-intercept b of the perpendicular line.
y=-1/5x+b
0=-1/5( -3/5) +b
â–Ľ
Solve for b
0=1/5* 3/5 +b
0=3/25+b
-3/25=b
b=-3/25
Now that we have the y-intercept, we can complete the equation. y=-1/5x+(-3/25) ⇔ y=-1/5x- 3/25 The line given by this equation is both perpendicular to y=5x+3 and passes through the point (- 35,0).