What must be true about the slopes of two perpendicular lines, neither of which is vertical?

The slopes are equal.

The slopes have product 1.

The slopes have product - 1.

One of the slopes must be 0.

Step-by-step explanation:

Perpendicular slopes are opposite reciprocals of each other. If one line has a slope of 2, then a line perpendicular to that line has a slope of - 1/2.

1 * - 1/2 = - 1

So the slopes have a product of - 1. Always.

The slopes have a product of - 1

Step-by-step explanation:

If the slopes are equal, they must be coinciding (the same)

The slopes for perpendicular lines have to be negative and reciprocal

in which case, if you multiply them, they should get negative 1

For example:

4 and - 1/4 are the slops of two perpendicular lines

4*-1/4=-1

Now, one of them can't be zero, that's not possible

If the product of the lines turn out to be 1, then they're not perpendicular.