Intermediate Geometry : How to find the slope of a perpendicular line

Study concepts, example questions & explanations for Intermediate Geometry

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Example Questions

Example Question #1 : Perpendicular Lines

Any line that is perpendicular to \(\displaystyle 18-9y=13x\) must have a slope of what?

Possible Answers:

\(\displaystyle -13/9\)

\(\displaystyle 1/2\)

\(\displaystyle 13/9\)

\(\displaystyle 9/13\)

\(\displaystyle -9/13\)

Correct answer:

\(\displaystyle 9/13\)

Explanation:

Two lines are perpendicular if and only if their slopes are negative reciprocals of each other. To find the slope, we must put the equation into slope-intercept form,  \(\displaystyle y=mx+b\), where \(\displaystyle m\) equals the slope of the line. First, we must subtract \(\displaystyle 18\) from both sides of the equation, giving us \(\displaystyle -9y=13x-18\). Next, we must divide both sides by \(\displaystyle -9\), giving us \(\displaystyle y=-13x/9+2\). We can now see that the slope of this line is \(\displaystyle -13/9\). Therefore, any line that is perpendicular to this one must have a slope of \(\displaystyle 9/13\).

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

What is the slope of any line perpendicular to \(\displaystyle y=-\frac{1}{2}x+5\)?

Possible Answers:

\(\displaystyle 2\)

\(\displaystyle -2\)

\(\displaystyle 5\)

\(\displaystyle -\frac{1}{2}\)

\(\displaystyle \frac{1}{2}\)

Correct answer:

\(\displaystyle 2\)

Explanation:

The slope of a perpendicular line is the negative reciprocal of the original slope.

The original slope is \(\displaystyle -\frac{1}{2}\).

The negative reciprocal of the original slope is:

\(\displaystyle -\frac{1}{-(\frac{1}{2})} = 2\)

Example Question #2 : How To Find The Slope Of A Perpendicular Line

What is the slope of a line perpendicular to \(\displaystyle x=100\)?

Possible Answers:

\(\displaystyle 0\)

\(\displaystyle \frac{1}{100}\)

\(\displaystyle -100\)

\(\displaystyle -\frac{1}{100}\)

\(\displaystyle 100\)

Correct answer:

\(\displaystyle 0\)

Explanation:

The equation \(\displaystyle x=100\) is a vertical line on the coordinate plane. Any line perpendicular to a vertical line is a horizontal line. Horizontal lines have slopes of zero.

The correct answer is \(\displaystyle 0\).

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

Assume the points \(\displaystyle (1,1)\) and \(\displaystyle (2,3)\) form a line. A perpendicular line is drawn to intersect this line. What is the slope of the perpendicular line?

Possible Answers:

\(\displaystyle \frac{1}{2}\)

\(\displaystyle -\frac{1}{2}\)

\(\displaystyle -2\)

\(\displaystyle 2\)

\(\displaystyle -1\)

Correct answer:

\(\displaystyle -\frac{1}{2}\)

Explanation:

Write the slope formula and find the slope of the first line.

\(\displaystyle m=\frac{y_2-y_1}{x_2-x_1}=\frac{3-1}{2-1}=\frac{2}{1}=2\)

The slope of the perpendicular line is the negative reciprocal of the original slope.

\(\displaystyle -\frac{1}{(2)} =-\frac{1}{2}\)

Therefore, the slope of the perpendicular line is \(\displaystyle -\frac{1}{2}\).

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

A line goes through the following \(\displaystyle (4,5)\) and \(\displaystyle (10,8)\).

Find the slope of a line perpendicular to the line given.

Possible Answers:

\(\displaystyle -2\)

\(\displaystyle 2\)

\(\displaystyle 1.5\)

\(\displaystyle 0.5\)

\(\displaystyle -0.5\)

Correct answer:

\(\displaystyle -2\)

Explanation:

The slope of a line perpendicular is the negative reciprocal of the given line.

Since the given slope is calculated as \(\displaystyle \frac{8-5}{10-4}=\frac{3}{6}=\frac{1}{2}\), then the slope of the perpendicular line would be  \(\displaystyle (-)2=-2\).

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