Most of us remember the Pythagorean theorem by its famous equation, a^2+b^2=c^2. The Greeks, as a relationship
between areas. This is how Euclid stated it: in all right-angled triangles the square on the side subtending the right angle is equalto the squares on the sides containing the right angle. That is, the area of the square built on the hypotenuse ("the side subtending the right angle") is equal to the combined area of the squares built on the other two sides.