∫tan^nxdx=∫tan^（n-2)x·(sec²x-1)dx
=∫tan^（n-2)x·sec²xdx-∫tan^（n-2)xdx
=∫tan^（n-2)x·dtanx-∫tan^（n-2)xdx
=[tan^(n-1)x]/(n-1)-∫tan^（n-2)xdx