**The Pythagorean theorem states that when a triangle has a right angle and all three sides are squared, the longest side squared will equal the size of the smaller two sides squared and summed.** It is usually expressed as a^2+b^2=c^2.

In the formula, the longest side is represented by the letter "c." This side is called the "hypotenuse." The other two sides are represented by "a" and "b."

A right angle is a 90-degree angle. A triangle with such an angle is called a right triangle. An example of the Pythagorean theorem in action is that if a right triangle has two sides which are 3 and 4 centimeters long, the hypotenuse will be 5 centimeters long. This is known because 3 squared is 9 and 4 squared is 16. When added together, the answer is 25. That is the size of the square of the hypotenuse, meaning if one finds the square root, one will obtain the length of the hypotenuse, in this case 5 centimeters.

The theorem is named after Pythagoras, a mathematician from ancient Greece. He is credited with creating the proof, although many argue that knowledge of the theorem predates Pythagoras. The theorem has numerous proofs.