ACT Math : How to find the equation of a perpendicular line

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #1 : How To Find The Equation Of A Perpendicular Line

What line is perpendicular to x + 3y = 6 and travels through point (1,5)?

Possible Answers:

y = 6x – 3

y = –1/3x – 4

y = 2x + 1

y = 3x + 2

y = 2/3x + 6

Correct answer:

y = 3x + 2

Explanation:

Convert the equation to slope intercept form to get y = –1/3x + 2.  The old slope is –1/3 and the new slope is 3.  Perpendicular slopes must be opposite reciprocals of each other:  m1 * m2 = –1

With the new slope, use the slope intercept form and the point to calculate the intercept: y = mx + b or 5 = 3(1) + b, so b = 2

So y = 3x + 2

Example Question #1 : How To Find The Equation Of A Perpendicular Line

What line is perpendicular to \(\displaystyle 3x + 5y = 15\) and passes through \(\displaystyle (3,2)\)?

Possible Answers:

\(\displaystyle y = \frac{1}{3}x - 5\)

\(\displaystyle y = \frac{-3}{5}x + 2\)

\(\displaystyle y = \frac{5}{3}x - 3\)

\(\displaystyle y = \frac{2}{5}x + 4\)

\(\displaystyle y = \frac{3}{2}x - 2\)

Correct answer:

\(\displaystyle y = \frac{5}{3}x - 3\)

Explanation:

Convert the given equation to slope-intercept form.

\(\displaystyle 3x + 5y = 15\)

\(\displaystyle 5y=-3x+15\)

\(\displaystyle y=-\frac{3}{5}+3\)

The slope of this line is \(\displaystyle -\frac{3}{5}\). The slope of the line perpendicular to this one will have a slope equal to the negative reciprocal.

The perpendicular slope is \(\displaystyle \frac{5}{3}\).

Plug the new slope and the given point into the slope-intercept form to find the y-intercept.

\(\displaystyle 2 = \frac{5}{3}(3) + b\)

\(\displaystyle 2=5+b\)

\(\displaystyle b = -3\)

So the equation of the perpendicular line is \(\displaystyle y = \frac{5}{3}x - 3\).

Example Question #1 : How To Find The Equation Of A Perpendicular Line

What is the equation of a line that runs perpendicular to the line 2x + = 5 and passes through the point (2,7)?

Possible Answers:

x/2 + y = 5

2x + y = 7

x/2 – y = 6

x/2 + y = 6

2x – y = 6

Correct answer:

x/2 + y = 6

Explanation:

First, put the equation of the line given into slope-intercept form by solving for y. You get y = -2x +5, so the slope is –2. Perpendicular lines have opposite-reciprocal slopes, so the slope of the line we want to find is 1/2. Plugging in the point given into the equation y = 1/2x + b and solving for b, we get b = 6. Thus, the equation of the line is y = ½x + 6. Rearranged, it is –x/2 + y = 6.

Example Question #1 : How To Find The Equation Of A Perpendicular Line

Line m passes through the points (1, 4) and (5, 2). If line p is perpendicular to m, then which of the following could represent the equation for p?

Possible Answers:

4x  3y = 4

2x + y = 3

x  y = 3

2x  y = 3

3x + 2y = 4

Correct answer:

2x  y = 3

Explanation:

The slope of m is equal to   y2-y1/x2-x1  =  2-4/5-1 -1/2                                  

Since line p is perpendicular to line m, this means that the products of the slopes of p and m must be 1:

 

(slope of p) * (-1/2) = -1

               

Slope of p = 2

So we must choose the equation that has a slope of 2. If we rewrite the equations in point-slope form (y = mx + b), we see that the equation 2x  y = 3 could be written as y = 2x – 3. This means that the slope of the line 2x – y =3 would be 2, so it could be the equation of line p. The answer is 2x – y = 3.

Example Question #1 : How To Find The Equation Of A Perpendicular Line

What is the equation for the line that is perpendicular to \(\displaystyle 4x - 3y = 6\) through point \(\displaystyle (4, 6)\)?

Possible Answers:

\(\displaystyle y = \frac{-3}{4}x +9\)

\(\displaystyle y = \frac{4}{3}x - 5\)

\(\displaystyle y = \frac{4}{3}x + 6\)

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

\(\displaystyle y = \frac{1}{3}x - 3\)

Correct answer:

\(\displaystyle y = \frac{-3}{4}x +9\)

Explanation:

Perpendicular slopes are opposite reciprocals.

The given slope is found by converting the equation to the slope-intercept form.

\(\displaystyle 4x - 3y = 6\)

\(\displaystyle y = \frac{4}{3}x - 2\) 

The slope of the given line is \(\displaystyle m = \frac{4}{3}\) and the perpendicular slope is  \(\displaystyle m = \frac{-3}{4}\).

We can use the given point and the new slope to find the perpendicular equation. Plug in the slope and the given coordinates to solve for the y-intercept.

\(\displaystyle 6 = \frac{-3}{4}(4) + b\)

\(\displaystyle b = 9\)

Using this y-intercept in slope-intercept form, we get out final equation: \(\displaystyle y = \frac{-3}{4}x + 9\).

Example Question #1 : How To Find The Equation Of A Perpendicular Line

Which line below is perpendicular to \(\displaystyle 5x+6y=18\)?

Possible Answers:

\(\displaystyle y = \frac{6}{5}x + 3\)

\(\displaystyle y=\frac{5}{6}x+\frac{6}{5}\)

\(\displaystyle y=-\frac{5}{6}x+\frac{6}{5}\)

\(\displaystyle y=\frac{5}{6}x+2\)

\(\displaystyle y=-\frac{6}{5}x+8\)

Correct answer:

\(\displaystyle y = \frac{6}{5}x + 3\)

Explanation:

The definition of a perpendicular line is one that has a negative, reciprocal slope to another.

For this particular problem, we must first manipulate our initial equation into a more easily recognizable and useful form: slope-intercept form or \(\displaystyle y=mx+b\).

\(\displaystyle 5x+6y=18\)

\(\displaystyle 6y=-5x+18\)

\(\displaystyle y=-\frac{5}{6}x+6\)

According to our \(\displaystyle y=mx+b\) formula, our slope for the original line is \(\displaystyle -\frac{5}{6}\). We are looking for an answer that has a perpendicular slope, or an opposite reciprocal. The opposite reciprocal of \(\displaystyle -\frac{5}{6}\) is \(\displaystyle \frac{6}{5}\). Flip the original and multiply it by \(\displaystyle -1\)

Our answer will have a slope of \(\displaystyle \frac{6}{5}\). Search the answer choices for \(\displaystyle \frac{6}{5}\) in the \(\displaystyle m\) position of the \(\displaystyle y=mx+b\) equation.

\(\displaystyle \dpi{100} y = \frac{6}{5}x + 3\) is our answer. 

(As an aside, the negative reciprocal of 4 is \(\displaystyle -\frac{1}{4}\). Place the whole number over one and then flip/negate. This does not apply to the above problem, but should be understood to tackle certain permutations of this problem type where the original slope is an integer.)

Example Question #3 : How To Find The Equation Of A Perpendicular Line

If a line has an equation of 2y=3x+3\(\displaystyle 2y=3x+3\), what is the slope of a line that is perpendicular to the line?

Possible Answers:

3\(\displaystyle 3\)

-\frac{2}{3}\(\displaystyle -\frac{2}{3}\)

-2\(\displaystyle -2\)

-\frac{3}{2}\(\displaystyle -\frac{3}{2}\)

\frac{3}{2}\(\displaystyle \frac{3}{2}\)

Correct answer:

-\frac{2}{3}\(\displaystyle -\frac{2}{3}\)

Explanation:

Putting the first equation in slope-intercept form yields y=\frac{3}{2}x+\frac{3}{2}\(\displaystyle y=\frac{3}{2}x+\frac{3}{2}\).

A perpendicular line has a slope that is the negative inverse. In this case, -\frac{2}{3}\(\displaystyle -\frac{2}{3}\).

Example Question #1 : How To Find The Equation Of A Perpendicular Line

Which of the following is possibly a line perpendicular to \(\displaystyle 9y=3x + 22\)?

Possible Answers:

\(\displaystyle 4y - 12x = 31\)

\(\displaystyle 5x - 2y = 18\)

\(\displaystyle 4y - 8x = 2\)

\(\displaystyle 3y + 9x = 54\)

\(\displaystyle 3x + 2y =34\)

Correct answer:

\(\displaystyle 3y + 9x = 54\)

Explanation:

To start, begin by dividing everything by \(\displaystyle 9\), this will get your equation into the format \(\displaystyle y=mx+b\).  This gives you:

\(\displaystyle y=\frac{1}{3}x+\frac{22}{9}\)

Now, recall that the slope of a perpendicular line is the opposite and reciprocal slope to its mutually perpendicular line.  Thus, if our slope is \(\displaystyle \frac{1}{3}\), then the perpendicular line's slope must be \(\displaystyle -3\).  Thus, we need to look at our answers to determine which equation has a slope of \(\displaystyle -3\).  Among the options given, the only one that matches this is \(\displaystyle 3y + 9x = 54\).  If you solve this for \(\displaystyle y\), you will get:

\(\displaystyle y = -3x + \frac{18}{3}\)

Example Question #1 : How To Find The Equation Of A Perpendicular Line

Which of the following is the equation of a line perpendicular to the line given by:

\(\displaystyle y = -3x +9\)?

Possible Answers:

\(\displaystyle y = x + 4\)

\(\displaystyle y = \frac{1}{3}x + 2\)

\(\displaystyle y = 3x - 9\)

\(\displaystyle y = -3x + 6\)

\(\displaystyle y = -3x - 9\)

Correct answer:

\(\displaystyle y = \frac{1}{3}x + 2\)

Explanation:

For two lines to be perpendicular their slopes must have a product of \(\displaystyle -1\).
\(\displaystyle -3*\frac{1}{3} = -1\) and so we see the correct answer is given by \(\displaystyle y = \frac{1}{3}x + 2\)

Example Question #1 : How To Find The Equation Of A Perpendicular Line

What is the equation of a line perpendicular to the line defined by the equaiton:

\(\displaystyle y = -9x + 2\)

Possible Answers:

\(\displaystyle y = \frac{1}{9}x +6\)

\(\displaystyle y = -9x + 1\)

\(\displaystyle y = -\frac{1}{9}x -2\)

\(\displaystyle y = -9x + 3\)

\(\displaystyle y = 9x - 13\)

Correct answer:

\(\displaystyle y = \frac{1}{9}x +6\)

Explanation:

Perpendicular lines have slopes whose product is \(\displaystyle -1\).

Looking at our equations we can see that it is in slope-intercept form where the m value represents the slope of the line,

\(\displaystyle y=mx+b\).

In our case we see that

\(\displaystyle y = -9x + 2\) therefore, \(\displaystyle m=-9\).

Since 

\(\displaystyle -9*\frac{1}{9} = -1\) we see the only possible answer is 

\(\displaystyle y = \frac{1}{9}x +6\).

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