用分部积分法来做
∫ (lnx)/x^2 dx
= -∫ (lnx) d(1/x)
= -lnx /x +∫ 1/x d(lnx)
= -lnx /x +∫ 1/x^2 dx
= -lnx /x -1/x
= -(1+lnx)/x +C (C为常数)