∫3x^4/(1+x^2)dx
=3∫(x^4-1+1)/(1+x^2)dx
=3∫[(x^2+1)(x^2-1)+1]/(1+x^2)dx
=3∫[(x^2-1)+1/(1+x^2)]dx
=x^3-3x+3arctanx+C