f(x)=ln(x/(x^2+1))
For f to be defined in RR we need x/(x^2+1)>0
x^2+1 is always >0 so we need x>0
As a result Domain is D_f=(0,+oo)
For xinD_f:
f'(x)=1/(x/(x^2+1))*(x/(x^2+1))'=((x^2+1)/x)(x/(x^2+1))' =
((x^2+1)/x)(((x)'(x^2+1)-x(x^2+1)')/(x^2+1)^2) =
((x^2+1)/x)((x^2+1-2x^2)/(x^2+1)^2) =
((x^2+1)/x)((-x^2+1)/(x^2+1)^2) =
-(cancel(x^2+1)/x)((x^2-1)/(x^2+1)^cancel(2)) =
-(x^2-1)/(x(x^2+1)) = -(x^2-1)/(x^3+x)