dy={arctan[(1-x^2)/(1+x^2)]}'dx
={1/(1+[(1-x^2)/(1+x^2)]^2)}x{[(1-x^2)/(1+x^2)]}'
化简得dy=[-2x/(1+x^4)]dx
耐心计算(1-x^2)/(1+X^2)的导数和化简一下就可以了