Mathematics for IGCSE & O level - Coordinate Geometry (Section 3 - No. 12)
If two lines are perpendicular, what is the relationship between their gradients?
The product of the gradients is 1.
The gradients are the same.
The product of the gradients is -1.
The gradients are undefined.
Explication
Perpendicular lines have gradients that multiply to give -1.