d(1+x^2)=2xdx
∫[3x/(1+x^2)]dx
=3/2*∫[2x/(1+x^2)]dx
=3/2*∫d(1+x^2)/(1+x^2)
=3/2ln|1+x^2|+C
则∫[上限1,下限0][3x/(1+x^2)]dx
=3/2ln2-3/2ln1
=3/2ln2