Question

Why does a horizontal have a slope of 0, but a vertical line has an undefined slope?

Answer 1

Answer: The slope of a vertical line. A vertical line has undefined slope because all points on the line have the same x-coordinate. As a result the formula used for slope has a denominator of 0, which makes the slope undefined.

A horizontal line has slope 0 because all its points have the same y-coordinate. As a result, the formula used for slope evaluates to 0.

Hope that helps!