#### Summary

The distance between the points $$P(x_1, y_1)$$ and $$Q(x_2, y_2)$$ is given by:

\begin{align}PQ^2 &= (x_2 - x_1)^2 + (y_2 - y_1)^2\\\\ \hspace{-3em}\text{or}\\\\ PQ &= \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}\end{align}

This is Pythagoras' theorem.