[-6（x+y)^2(x-y)^3]/[3(x+y)(x-y)^2]^2
=-2/[3(x-y)]能不能具体点？？[-6(x+y)^2(x-y)^3]/[9(x+y)^2(x-y)^4],(x+y)^2约掉，(x-y)^3和(x-y)^4约剩余(x-y),-6和9约剩-2和3最后为-2/[3(x-y)]