sin2a=2tana/[1+tan平方a]
2tana/[1+tan平方a
=2(sina/cosa)/[1+sin&2a/cos^2a]
=2(sina/cosa)/[(sin^2a+cos^2a)/cos^2a]
=2(sina/cosa)*cos^2a
=2sinacosa
=sin2a
求证：sin2a=2tana/tan平方a
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