先化简,再求值：y(x-y)-x(x+y)/x平方-y平方除以x平方+y平方/x+y,其中x=2,y=1[y(x-y)-x(x+y)]/(x^2-y^2)÷（x^2+y^2)/x+y=(yx-y^2-x^2-xy)/(x+y)(x-y)*(x+y)/(x^2+y^2)=-(x^2+y^2)/(x+y)(x-y)*[(x+y)/(x^2+y^2)]=-1/(x-y)=-1
