Given f(x)=arctan(1−x1+x)f(x) = \arctan(\frac{1-x}{1+x})f(x)=arctan(1+x1−x), what is the derivative f′(x)f'(x)f′(x)?
1/(1+x^2)
-1/(1+x^2)
1/(1-x^2)
-(1+x^2)