f=2x1x2-2x1x3-6x2x3
= 2(y1+y2)(y1-y2)-2(y1+y2)y3-6(y1-y2)y3
= 2y1^2 - 8y1y3 - 2y2^2 + 4y2y3
= 2(y1-2y3)^2 - 2y2^2 + 4y2y3 - 8y3^2
= 2(y1-2y3)^2 - 2(y2-y3)^2 - 6y3^2
= 2z1^2-2z2^2-6z3^2
f=2x1x2-2x1x3-6x2x3
= 2(y1+y2)(y1-y2)-2(y1+y2)y3-6(y1-y2)y3
= 2y1^2 - 8y1y3 - 2y2^2 + 4y2y3
= 2(y1-2y3)^2 - 2y2^2 + 4y2y3 - 8y3^2
= 2(y1-2y3)^2 - 2(y2-y3)^2 - 6y3^2
= 2z1^2-2z2^2-6z3^2