i don't know what are the techniques in integrating in calculus

The answer given is a very nice application of integration by parts (see http://en.wikipedia.org/wiki/Integration_by_parts). Bravo for that! But it uses the derivative of the arctangent (which is not derived in the solution presented) - but you can find it at: http://www.coastal.edu/mathcenter/HelpPages/Handouts/invhyp.PDF

But you could also simply look up the formula for the artan integral at http://en.wikipedia.org/wiki/List_of_integrals_of_inverse_trigonometric_functions