The Pythagorean theorem deals with right triangles: Simply stated, the sum of the squares of the legs equals the square of the hypotenuse (longest side). The converse of the Pythagorean theorem is also true: If the sum of the squares of a triangle’s two sides equals the square of the longest side, then the triangle is a right triangle. (For an illustration of the Pythagorean theorem, see “History of Mathematics.”)