1. The problem statement, all variables and given/known data Find the derivative of arctan[(1 - x) / (1 + x)]. 2. Relevant equations Everything in the "Show Steps" section: http://www.wolframalpha.com/input/?i=derivative+arctan[(1+-+x)+/+(1+++x)] My problem is that I don't know how Wolfram manages to simplify: - 2 / [1 + ((1 - x) / (1 + x))^2](1 + x)^2 ^ which is also what I've managed to get, to get , - 1 / (1+x^2) 3. The attempt at a solution Everything you see Wolfram does pretty much.