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

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 $L_2$ :


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

Also presented as
This can also be presented in the elegant form:


 * $\mu_1 \mu_2 + 1 = 0$

or:


 * $\mu_1 \mu_2 = -1$