f = (x1+x2-x3)^2 + (x2)^2-(x3)^2+2x2x3= (x1+x2-x3)^2 + (x2+x3)^2 - 2(x3)^2= (y1)^2 + (y2)^2 - 2(y3)^2y1 = x1+x2-x3, y2 = x2+x3, y3 = x3
