x/(x+y)+y/(x-y)-2xy/(x^2-y^2)=x*(x-y)/(x+y)(x-y)+y*(x+y)-2xy/(x^2-y^2)=(x^2-xy)/(x^2-y^2)+(xy+y^2)/(x^2-y^2)-2xy/(x^2-y^2)=(x^2+y^2-2xy)/(x^2-y^2)=(x-y)/(x+y) 代入X=5,Y=2得（5-2）/（5+2）=3/7