Question

If the two lines with slope m1 and m2 are perpendicular then their slopes has relation ______

(a) m1 + m2 = 1

(b) m1 * m2 = 1

(c) m1 * m2 = -1

(d) m1 + m2 = -1

This question was addressed to me in homework.

My query is from Various Forms of the Equation of a Line topic in section Straight Lines of Mathematics – Class 11

Answer 1

The correct answer is (c) m1 * m2 = -1

To explain: If the two lines are perpendicular then if one line form angle α with positive x-axis then the other line form angle 90° + α.

If m1 = tan α then m2 will be tan (90°+ α) = – cot α = -1/tan α

=> m1 * m2 = – 1.