## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}$
For two points $P_1$ and $P_2$ with coordinates of $(x_1,y_1)$ and $(x_2,y_2)$ respectively, the distance between them is denoted by $d$ where: $d=\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}$