Condition for Straight Lines in Plane to be Perpendicular/Slope Form/Proof 2

Theorem

Let $L_1$ and $L_2$ be straight lines in the Cartesian plane.

Let the slope of $L_1$ and $L_2$ be $\mu_1$ and $\mu_2$ respectively.

Then $L_1$ is perpendicular to $L_2$ if and only if:

$\mu_1 = -\dfrac 1 {\mu_2}$

Proof

This is a special case of Slope of Orthogonal Curves.

$\blacksquare$