x*x*y*y-20x*y+x*x+y*y+81
=[(xy)*(xy)-18xy+81]+[x*x-2xy+y*y]
=(xy-9)*(xy-9)+(x-y)*(x-y)
=0
两个数的平方和等于0,则这两个数同时等于0
xy-9=0且x-y=0
解这个方程组可得：
x=3,y=3或x=-3,y=-3