原题中-2y一项应该掉了一个平方.(x+2y+m)(2x-y+n)=2x 2 -xy+nx+4xy-2y 2 +2ny+2mx-my+mn =2x 2 +3xy-2y 2 +(n+2m)x+(2n-m)y+mn =2x+3xy-2y 2 -x+8y-6 故n+2m=-1,2n-m=8,mn=-6 解得m=-2,n=3 代入后式即可求得值.