（1）x/（x+y)-x/(x-y)=x(x-y)/(x+y)(x-y)-x(x+y)/(x+y)(x-y)=(x^2-xy-x^2-xy)/(x+y)(x-y)=-2xy/(x+y)(x-y)（2）a+2+4/(a+2 )=(a+2)(a+2)/(a+2)+4/(a+2)=(a^2+4a+4+4)/(a+2)=(a^2+4a+8)/(a+2)貌似(2)a+2+4/(a-2 )=(a...