The Pythagorean Theorem is what you probably mean. It is basically to solve for the sides of a triangle. The equation is a^2+b^2=c^2. Plug in at least two of the variables and you will get any side.
I believe you are asking about the Pythagorean theorem? If you are, here's what it is: In mathematics, the Pythagorean theorem, aka Pythagoras's theorem, is a fundamental relation in Euclidean geometry among the three sides of a right triangle. It says that the square of the hypotenuse (the side opposite the right angle) is equivalent to the sum of the squares of the two other sides.
