tan(2a)=tan(a+a)= (tan(a)+tan(a)) / (1-tan(a)*tan(a)) =2tana/[1-(tana)'2]
正切二倍角公式：tan2α = 2tanα/[1 - (tanα)^2] tan(1/2*α)=(sin α)/(1+cos α)=(1-cos α)/sin α
tan(2a)=tan(a+a)= (tan(a)+tan(a)) / (1-tan(a)*tan(a)) =2tana/[1-(tana)'2]
正切二倍角公式：tan2α = 2tanα/[1 - (tanα)^2] tan(1/2*α)=(sin α)/(1+cos α)=(1-cos α)/sin α
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