My favorite mathematical theorem would be the Pythagoras Theorem. This theorem shows that the square of the hypotenuse in a right triangle is the sum of the the square of the other two sides.
$ a^2 = b^2 + c^2 $ where a is the hypotenuse of the triangle and b and c are the two other sides.
This is a simple formula which is easy to remember but very useful. This theorem is also one that can be easily proved within a few minutes.
How Jian Yoong
