x^2-y分之y(x-y)-x(x+y)除以x+y分之x^2+y^2.其中x=2,y=-1.先化简再求值.

x^2-y分之y(x-y)-x(x+y)除以x+y分之x^2+y^2=[(-x^2-y^2)/(x^2-y)]÷[(x^2+y^2)/(x+y)]=(x+y)/(y-x^2)=(2-1)/(-1-2^2)=-1/5如果此题是x^2-y^2分之y(x-y)-x(x+y)除以x+y分之x^2+y^2=[(-x^2-y^2)/(x^2-y^2)]÷[(x^2+y^2...