The Pythagorean theorem states, in words, that the sum of the squares of the legs of a right triangle is equal to the square of the hypotenuse. Translating the theorem, a and b are the legs of the right triangle and c is the hypotenuse. To use the legs to find the length of the hypotenuse, substitute them into a and b. Square both of these numbers. Add them together. This gives us c²; in order to just find c, we must take the square root of the sum. This will give us the length of the hypotenuse.
