x的-2次方-y的-2次方分之(x的-1次方+y的-1次方)的平方
=x^(-2)-[x^(-1)+y^(-1)]^2/y^(-2)
=1/x²-[1/x+1/y]²/(1/y²)
=1/x²-[(x+y)/xy]²y²
=1/x²-[(x+y)²/x²]
=[1-(x+y)²]/x²
=(1+x+y)(1-x-y)/x²