中等math-u06-d03
Two non-vertical straight lines are perpendicular when:
A. The product of their slopes is -1✓ 答案
B. Their slopes are equal
C. Their slopes add to 1
D. Their slopes add to 0
解說
A perpendicular gradient is the negative reciprocal, so m1 x m2 = -1. Two horizontal lines have product 0 and are parallel, not perpendicular.