source 2008 Sutherland assume ZZ1 = 1 assume ZZZ1 = 1 assume ZZ2 = 1 assume ZZZ2 = 1 compute P = X2-X1 compute R = Y2-Y1 compute PP = P^2 compute PPP = P PP compute Q = X1 PP compute X3 = R^2-PPP-2 Q compute Y3 = R (Q-X3)-Y1 PPP compute ZZ3 = PP compute ZZZ3 = PPP