When two nonhorizontal, nonvertical lines are perpendicular, what is true about their slopes?

• A. The sum of the slopes is zero
• B. The product of the slopes is -1
• C. The slopes are equal
• D. One slope is zero

Answer: (B)

Perpendicular lines have slopes that are negative reciprocals of each other. If one line has slope m, the other must have slope -1/m. Multiplying these gives m × (-1/m) = -1, so the product of the slopes is -1. Since both lines are nonhorizontal and nonvertical, both slopes are defined and finite, so this relationship applies.

The other statements don’t fit because: the sum of slopes being zero would require the slopes to be exact negatives of each other in every case, which isn’t the general condition for perpendicularity; equal slopes mean the lines are parallel, not perpendicular; and if one slope were zero (horizontal), the other would have to be undefined (vertical), which isn’t allowed here.