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

Since we have a right triangle with legs $a$, $b$ and the hypotenuse $c$, then by the Pythagorean theorem, we have $$c^2=a^2+b^2.$$ Thus, the hypotenuse is given by $$c=\sqrt{a^2+b^2}=\sqrt{6^2+8^2}=\sqrt{100}=10 .$$