问题标题：
tan(π/4+α)-tan(π/4-α)=2tan2α
问题描述：
tan(π/4+α)-tan(π/4-α)=2tan2α
陈亚绒回答：
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
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