x^2y^2-20xy+x^2+y^2+81=(x^2y^2-18xy+81)+(x^2+y^2-2xy)=(xy-9)^2+(x-y)^2=0两个完全平方相加=0则 xy-9=0且x-y=0解法一：则x=y=3或x=y=-3则x+y=6或-6解法二：则(x+y)^2=(x-y)^2+4xy=0+4*9=36则x+y=6或-6...