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

$D=\sqrt {(x_2−x_1)^2+(y_2−y_1)^2}$
The distance $D$ between two points $P_1(x_1,y_1)$ and $P_2(x_2, y_2)$ can be defined by the Pythagorean theorem. Therefore, the distance between $P_1$ and $P_2$ is given by the formula: $D=\sqrt {(x_2−x_1)^2+(y_2−y_1)^2}$