Which Side Is c?

First find the hypotenuse. It is the side opposite the right angle. Call it c in a^2 + b^2 = c^2. To find the hypotenuse, add the squares of the two legs. To find a leg, subtract the square of the other leg from the square of the hypotenuse. Then take the positive square root and check your arithmetic.

Two checks catch most mistakes. If the hypotenuse is known, the leg you find must be shorter than it. A leg of 14 from a hypotenuse of 10 means the squares were added instead of subtracted. If the unknown side is the hypotenuse, it must come out longer than either leg. For example, with a hypotenuse of 13 and a leg of 5, the other leg is 12. The leg 12 is less than 13, and 5^2 + 12^2 = 169 = 13^2 confirms it.