令 x1=y1+y2, x2=y1-y2, 则
f = (y1+y2)(y1-y2)+(y1+y2)y3+(y1+y2)y4+(y1-y2)y3+(y1-y2)y4+y3y4
= y1^2-y2^2+2y1y3+2y1y4+y3y4
= (y1+y3+y4)^2-y2^2-y3^2-y4^2-y3y4
= (y1+y3+y4)^2-y2^2-(y3+(1/2)y4)^2-(3/4)y4^2
= z1^2-z2^2-z3^2-(3/4)z4^2.
令 x1=y1+y2, x2=y1-y2, 则
f = (y1+y2)(y1-y2)+(y1+y2)y3+(y1+y2)y4+(y1-y2)y3+(y1-y2)y4+y3y4
= y1^2-y2^2+2y1y3+2y1y4+y3y4
= (y1+y3+y4)^2-y2^2-y3^2-y4^2-y3y4
= (y1+y3+y4)^2-y2^2-(y3+(1/2)y4)^2-(3/4)y4^2
= z1^2-z2^2-z3^2-(3/4)z4^2.