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

Use Pythagorean theorem $a^{2}+b^{2}=c^{2}$ where $c$ is the length of the hypotenuse and $a$ and $b$ are the lengths of the two other sides. $3^{2}+4^{2}=c^{2} \\9 + 16 = c^{2} \\25 = c^{2}$ Take the principal square root of both sides : $\sqrt{25}=\sqrt{c^{2}} \\5=c$