81(x+y)²-121(m+n)² =（9x+9y-11m-11n)(9x+9y+11m+11n)(x²+y²)²-x²y² =(x²+xy+y²)(x²-xy+y²)(2m-n)²-(3m+2n)²=(2m-n+3m+2n)(2m-n-3m-2n)=-(5m+n)...