Originally Posted by

**Jones** Hi,

I got stuck while trying to calculate the third derivative for arctan.

The second derivative for arctan is $\displaystyle \frac{-2x}{(1+x^2)^2}$

No problem. Then i try to rewrite it as: $\displaystyle -2x*(1+x^2)^{-2} $ and use the product rule. This is what i end up with:

$\displaystyle -2x*(1+x^2)^{-2} $

$\displaystyle -2x*-2(1+x^2)^{-3} *2x - 2*(1+x^2)^{-2}$

which can be rearanged into: $\displaystyle -4x^2 -\frac{2}{(1+x^2)^3} -\frac{2}{(1+x^2)^2}$

and it's wrong. What am i doing wrong?