组合数公式啦，如图
解: 令 x1=y1+y2, x2=y1-y2,x3=y3,x4=y4f = y1^2-y2^2+y1y3-y2y3+y3y4= (y1+y3/2)^2-(y2+y3/2)^2+y3^2y3y4=z1^2-z2^2+z3z4=w1^2-w2^2+w3^2-w4^2X=AY
