what is the hypotenuse of a triangle that is?
A Right Triangle's Hypotenuse. The hypotenuse is the largest side in a right triangle and is always opposite the right angle. In the triangle on the left, the hypotenuse is the side AB which is opposite the right angle, $$ \angle C $$. Hypotenuse Calculator. Online tool calculates the hypotenuse (or a leg) using the Pythagorean theorem.