15
as
15²=12²+9²(using Pythagoras theorem)
or
225= 144+81
225=225
(the Pythagorean theorem, also known as Pythagoras's theorem, is a fundamental relation in Euclidean geometry among the three sides of a right triangle. It states that the square of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the other two sides. )
