A hypotenuse is the longest sie of a right triangle, the side opposite of the right angle. The length of the hypotenuse of a right triangle can be found using the Pythagorean theorem, which states that the square of the length of the hypotenuse equals the sum of the squares of the lengths of the two other sides. One method some teachers use to help kids remember 'hypotenuse': When you are in an airplane, and you have to go to the bathroom, you are using a High Pot, In Use.