what is the Pythagorean theorem

The Pythagorean Theorem is named for the Greek mathematician Pythagoras.

It uses the equation   $${{\mathtt{a}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{{\mathtt{b}}}^{{\mathtt{2}}} = {{\mathtt{c}}}^{{\mathtt{2}}}$$    to solve for lengths of a right triangle's sides, given two sides and one unknown.

A and b would be the lengths of the triangle's legs (two shorter sides) and c would be the length of the hypotenuse (longest side, diagonal to legs).

You can read more about it here:

http://www.purplemath.com/modules/pythagthm.htm