In a right triangle (one where one interior angle is 90°), the longest side is called the hypotenuse. It is always the side opposite the 90° 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 other two sides.
Hypotenuse = √ (side a2 + side b 2)
For Example: If one of the other sides has a length of 3 (when squared it is 9) and the other has a length of 4 (when squared it becomes 16), then their sum of squares add up to 25. The length of the hypotenuse is the square root of 25, that is, 5.
A right-angled triangle and its hypotenuse.