Question

The equation of line AB is (y-3) = 5 (x - 4). What is the slope of a line perpendicular to line AB?

A) -5
B) 5
C) -1/5
D) 1/5

Answer 1

Perpendicular lines are lines that intersect. The angle made by their intersection is a right angle. To know if 2 lines are perpendicular is through their slopes. The slope of the other line must be the negative reciprocal of the slope of the first. Since the slope of line AB is 5 hence the slope of the other line must be -1/5. C is the answer. 

Answer 2

y - 3 = 5(x - 4)
y - 3 = 5x - 20
y = 5x - 20 + 3 = 5x - 17
=> slope = 5

For perpendicular line, m2 = -1/m1 = -1/5