The Pythagorean Theorem expresses the relationships between the lengths of the sides of a right triangle.  For example, in the figure below the right triangle has sides labeled A, B, and C.  Here, C labels the longest side, called the hypotenuse.  

PythagoreanTheorem.gif (2233 bytes)

The Pythagorean Theorem states that for any right triangle,

C2 = A2 + B2.