Two lines are perpendicular when... the product of their slope is -1, the product of the slope is greater than 0, they have the same slope, or when the slope is undefined?

The answer is the product of their slope is -1.

First, let's break this down. Perpendicular lines are line with opposite reciprocal slopes. Opposite meaning the inverse sign and reciprocal meaning flipped.

For instance,

If a line had the slope of 5, its inverse would have a slope of -1/5.

Now, if we multiply these numbers together,

5 * -1/5 = -1

The 5's cancel out.

This works for any two perpendicular lines

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