∫ ( tan²x + tan⁴x) dx
= ∫ tan²x (1 + tan²x) dx
= ∫ tan²x sec²x dx
= ∫ tan²x d tanx
= (1/3) tan³x + c= ∫tan²x sec²x dx这步是怎么得出来的？1 + tan²x = cos²x / cos²x + sin²x / cos²x = (sin²x + cos²x)/cos²x = 1/cos²x = sec²x