#int_0^(tan^(-1) x)1/(1+tan^2 t) dt#
By Trigonometric Identities,
#=int_0^(tan^(-1) x)1/sec^2 t dt
=int_0^(tan^(-1) x)cos^2 t dt
=int_0^(tan^(-1)x)1/2(1+cos 2t)dt#
By integrating and #sin 2t=2sin t cos t#
#=1/2 [1+(sin 2t)/2]_0^(tan^(-1) x)
=1/2[t+sin t cos t]_0^(tan^(-1) x)#
#=1/2(tan^(-1) x +sin(tan^(-1) x) cdot cos(tan^(-1) x))#
#=1/2(tan^(-1) x+ x/sqrt(1+x^2) cdot 1/sqrt(1+x^2))#
#=1/2(tan^(-1) x+x/(1+x^2))#
I hope that this was clear.