Question

How do you integrate arctan(x dx? i think that if you simplify the integral you get:?

Answer 1

Integration by parts will help here. Letting $u=\arctan x$ and $\mathrm dv=\mathrm dx$, you end up with $\mathrm du=\dfrac{\mathrm dx}{1+x^2}$ and $v=x$. Now

$\displaystyle\int\arctan x\,\mathrm dx=uv-\int v\,\mathrm du$
$\displaystyle\int\arctan x\,\mathrm dx=x\arctan x-\int\frac x{1+x^2}\,\mathrm dx$

For the remaining integral, setting $y=1+x^2$ gives $\dfrac{\mathrm dy}2=x\,\mathrm dx$, so

$\displaystyle\int\frac x{1+x^2}\,\mathrm dx=\frac12\int\frac{\mathrm dy}y=\frac12\ln|y|+C=\frac12\ln(1+x^2)+C$

Putting everything together, you end up with

$\displaystyle\int\arctan x\,\mathrm dx=x\arctan x-\frac12\ln(1+x^2)+C$