∫1/(x^7+4x) dx
=1/4 *∫(x^6+4 -x^6)/(x^7+4x) dx
=1/4 *[∫1/x dx - ∫ x^6 /(x^7+4x) dx]
=1/4 *lnx -1/4* ∫ x^5 /(x^6+4) dx
=1/4 *lnx -1/24* ln(x^6+4) +C,C为常数