tan(π/4+α)-tan(π/4-α)=2tan2α
问题描述：
tan(π/4+α)-tan(π/4-α)=2tan2α
答
左式=(1+tanα)/(1-tanα)-(1-tanα)/(1+tanα)
=[(1+tanα)²-(1-tanα)²]/[(1-tanα)(1+tanα)]
=(4tanα)/(1-tan²α)
=2tan2α=右式