∫ (tanx + 2cotx)² dx,展开可以了
= ∫ (tan²x + 4tanxcotx + 4cot²x) dx
= ∫ tan²x dx + 4∫ dx + 4∫ cot²x dx
= ∫ (sec²x - 1) dx + 4∫ dx + 4∫ (csc²x - 1) dx
= ∫ sec²x dx - ∫ dx + 4∫ csc²x dx
= tanx - x - 4cotx + C