程國学 f=aux+ a22y2+a332+2a1xy+ 2a13xz+2a23yz 取 21=a12,a1=a13,432=a23, 从而,2a12xy=a12xy+a2, 2a13X=a13xz+a312X, 2a232=a2y2+a32y 第七章第七章 工 程 数 学 f = a11x 2 + a22y 2 + a33z 2 +2a12xy + 2a13xz + 2a23yz 取 a21 = a12 , a31 = a13 , a32 = a23 , 从而, 2a12xy = a12xy + a21yx , 2a13xz = a13xz + a31zx , 2a23yz = a23yz + a32zy