Given:
Slope is undefined.
To find:
Which graph has an undefined slope?
Solution:
We know that, slope is change in y-values with respect to x-values.

The slope is undefined if the denominator is 0.


It means, the x-values are same. It is possible only for a vertical line.
Therefore, the slope of vertical lines are undefined or we can say that graph of a vertical line has an undefined slope. It is of the form
, where, a is a constant and it can be any real number.