Pythagorean Theorem

In his book, 5000B.C. and Other Philosophical Fantasies, Professor R. Smullyan tells of an experiment he ran in one of his geometry classes. He drew a right triangle on the board with squares on the hypotenuse and legs and observed the fact that the square on the hypotenuse had a larger area than either of the other two squares. He asked, "Suppose these three squares were made of beaten gold, and you were offered either the one large square or the two small squares. Which would you choose?" Interestingly enough, about half the class opted for the one large square and half for the two small squares. Both groups were equally amazed when told that it would make no difference.

The Pythagorean Theorem (Pyth Thm) is the statement that the sum of the areas of the two small squares equals the area of the big one. 

In algebraic terms, a² + b² = c² where c is the hypotenuse while a and b are the legs of the triangle.

The theorem is of fundamental importance in Euclidean Geometry where it serves as a basis for the definition of distance between two points. It's so basic and well known that, I believe, anyone who took geometry classes in high school couldn't fail to remember it long after other math notions got thoroughly forgotten.