Let’s call the long side c (5), the middle side b (4), and the small side a (3). Our area equation for these triangles is: Area = F · hypotenuse2 where F is some area factor (6/25 or .24 in this case; the exact number doesn’t matter). Now let’s play with the equation: Area(Big) = Area(Medium) + Area(Small) F · c2 = F · b2 + F · a2 Divide by F on both sides and you get: c2 = b2 + a2