求证：1+2sina cosa /sin^2a -cos^2a = tana +1/tana -1

左式=(1+sin2α)/(-cos2α)（倍角公式） =(1+2tanα/(1+tan²α))/-(1-tan²α)/(1+tan²α))（万能公式） =((1+tan²α+2tanα)/-(1+tan²α))/(1-tan²α)/(1+tan²α)) =(1+tanα)²/-(1-tan²α) =(1+tanα)²/-((1