My favorite theorem is the Pythagorean theorom. Not necessarily because it is actually my favorite but more because of the memory behind the theorem. Ask me about the story in class, it's my favorite math memory at Purdue. Pythagorean Theorem
In any right triangle, the area of the square whose side is the hypotenuse (the side opposite the right angle) is equal to the sum of the areas of the squares whose sides are the two legs (the two sides that meet at a right angle).
