This can be proven using area.
For any right triangle, the Pythagorean Theorem can be used to calculate the length on an unknown side when the lengths of the two other sides are known. Also, if the Pythagorean Theorem is true for a triangle in which all sides are known, then it can be concluded that the triangle is a right triangle.
Draw four copies of one right triangle with legs and and hypotenuse Arrange them so they create a square.
Find the length of
Since the triangle is a right triangle, we can use the Pythagorean Theorem to find the length of