计算：x-y分之x×x+y分之y的平方-x的4次方-y的4次方分之x的4次方y除以x的平方+y的平方分之x的平方
=x/x-y分之x× y^2/x+y - x^4y/(x^4-y^4) * (x^2+y^2)/x^2
=xy^2/(x^2-y^2) - x^2y/(x^2-y^2)
=xy(y-x)/(x^2-y^2)
=-xy(x-y)/(x+y)(x-y)
=-xy/(x+y)