2x/(1-x ^2)的导数为何是2(1+x^2)/(1-x^2)^2
设m=2x,n=1-x^2
y'=(m/n)'=m'n-n'm=2(1-x^2)-(-2x)*2x-=2+2x^2
为啥正确答案会有个分母呢,不懂啊