使用分部积分法
∫ln(x/3) dx
=ln(x/3) *x - ∫ x * d ln(x/3)
=ln(x/3) *x - ∫ x * 3/x *1/3 dx
=ln(x/3) *x - ∫dx
=ln(x/3) *x - x +C ,C为常数