How to find the slope of perpendicular lines

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SAT Math › How to find the slope of perpendicular lines

Questions 1 - 10
1

What is the slope of the line perpendicular to the line represented by the equation y = -2x+3?

-2/3

-1/2

1/2

2/3

2

Explanation

Perpendicular lines have slopes that are the opposite of the reciprocal of each other. In this case, the slope of the first line is -2. The reciprocal of -2 is -1/2, so the opposite of the reciprocal is therefore 1/2.

2

What is a possible equation of a perpendicular line that intersects ?

Explanation

The line is a vertical line. The line perpendicular to a vertical line must always have a slope of zero.

The only valid answer with a slope of zero is:

3

What is the slope of any line perpendicular to 2_y_ = 4_x_ +3 ?

– 4

2

½

– ½

Explanation

First, we must solve the equation for y to determine the slope: y = 2_x_ + 3/2

By looking at the coefficient in front of x, we know that the slope of this line has a value of 2. To fine the slope of any line perpendicular to this one, we take the negative reciprocal of it:

slope = m , perpendicular slope = – 1/m

slope = 2 , perpendicular slope = – 1/2

4

What line is perpendicular to 2x + y = 3 at (1,1)?

2x + 3y = 1

x + 2y - 3

3x + 2y = 1

x – 2y = -1

Explanation

Find the slope of the given line. The perpendicular slope will be the opposite reciprocal of the original slope. Use the slope-intercept form (y = mx + b) and substitute in the given point and the new slope to find the intercept, b. Convert back to standard form of an equation: ax + by = c.

5

The equation of a line is: 8x + 16y = 48

What is the slope of a line that runs perpendicular to that line?

2

-2

8

-1/8

-1/4

Explanation

First, solve for the equation of the line in the form of y = mx + b so that you can determine the slope, m of the line:

8x + 16y = 48

16y = -8x + 48

y = -(8/16)x + 48/16

y = -(1/2)x + 3

Therefore the slope (or m) = -1/2

The slope of a perpendicular line is the negative inverse of the slope.

m = - (-2/1) = 2

6

What is the slope of the line perpendicular to the given line?

Explanation

Let's write the equation of the given line in slope-intercept form.

The slope of the given line is expressed by the coefficient of x. The slope here is .

To find the slope of the perpendicular line, we take the negative reciprocal of the given line's slope. Therefore, the slope of the perpendicular line is .

7

Solve each problem and decide which is the best of the choices given.

What is the slope of a line perpendicular to the following?

Explanation

A slope perpendicular to another line can be found by taking the reciprocal of the orignal slope and changing the sign.

If you solve for in the given equation,

divide by three on each side

.

There is a slope of because the equation is in slope-intercept form where m represents the slope,

.

The reciprocal of that with a changed sign is

.

8

Screen_shot_2013-09-04_at_10.56.34_am

What is the equation of a line perpendicular to the one above, passing through the point ?

y=-\frac{1}{3}x+\frac{8}{3}

y=-\frac{4}{3}x+2

y=\frac{4}{3}x-2

y=\frac{1}{3}x+\frac{4}{3}

y=-\frac{1}{2}x+3

Explanation

Looking at the graph, we can tell the slope of the line is 3 with a -intercept of , so the equation of the line is:

y=3x-4

A perpendicular line to this would have a slope of -\frac{1}{3}, and would pass through the point so it follows:

y=-\frac{1}{3}x+c\rightarrow 2=-\frac{1}{3}(2)+c\rightarrow c=8/3\rightarrow y=-\frac{1}{3}x+\frac{8}{3}

9

What is the slope of a line perpendicular to the following:

Explanation

The question puts the line in point-slope form y y1 = m(x x1), where m is the slope. Therefore, the slope of the original line is 1/2. A line perpendicular to another has a slope that is the negative reciprocal of the slope of the other line. The negative reciprocal of the original line is _–_2, and is thus the slope of its perpendicular line.

10

Line M passes through the points (2,2) and (3,–5). Which of the following is perpendicular to line M?

y = 7_x_ – 6

y = –7_x_ – 5

y = (1/7)x + 3

y = –(1/7)x – 1

y = 7_x_ + 4

Explanation

First we find the slope of line M by using the slope formula (_y_2 – _y_1)/(_x_2 – _x_1).

(–5 – 2)/(3 – 2) = –7/1. This means the slope of Line M is –7. A line perpendicular to Line M will have a negative reciprocal slope. Thus, the answer is y = (1/7)x + 3.

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