Use Exercise 66 to prove that two lines with nonzero slopes m₁ and m₂ are perpendicular if and only if m₂ = −1/m₁.

Data From Exercise 66

Let L₁ and L₂ be the lines of slope m₁ and m₂ [Figure 28(B)]. Show that the angle θ between L₁ and L₂ satisfies cot θ = m₂m₁ + 1/m₂ −m₁.

L2 L X