The document explains the Pythagorean theorem and how to use it to determine if a triangle is a right triangle. It provides the formula, A2 + B2 = C2, where C is the hypotenuse and A and B are the legs. It then works through examples of applying the formula to find the length of an unknown side of different right triangles. Finally, it explains the converse of the theorem, which is that if the sum of the squares of two sides equals the square of the third side, then the triangle must be a right triangle.