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Using Two Points to Find Slope                              Page 5 of 5

Undefined Slope

Vertical lines always have an undefined slope.

​A vertical line that goes straight up and down has an undefined slope.  With a vertical line, the run will be 0 because the x-values are not changing.  If you use two points on a vertical line and plug them into the formula for slope, you're going to end up with a 0 in the denominator of the fraction.  Since you can't divide by 0, we say that the slope is undefined.  Think of the word NO to help you remember:
Think of the word NO. You can't divide by 0! If 0 is in the denominator, it means the slope is undefined.

Example

Find the slope of the line through the points (3,-1) and (3,2).

​Step 1: Label the points. ​
How to label points 1 and 2 when using the formula for slope.

Step 2: Plug the values into the formula for slope.
How to use the formula for slope to find the slope of a vertical line.

Step 3: Make sure the slope is a simplified fraction.  There's a problem here!  You can't have a 0 in the denominator of a fraction.  This means the slope is undefined.
If there is a 0 in the denominator, the slope is undefined.

Practice

Ready to try using the formula for slope on your own? Click the START button below to take a practice quiz.
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