Chapter 1.1, Problem 49E

Let $\left(x_1,y_1\right)$ and $\left(x_2,y_2\right)$ be two points lying in the $xy$-plane. Show that the distance between the two points is given by

$d=\sqrt{{\left(x_2-x_1\right)}^2-{\left(y_2-y_1\right)}^2}$

Hint: Refer to the accompanying figure, and use the Pythagorean Theorem.