sin2a(1+tana*tana/2）
=2sinacosa[1+(sina/cosa)*(sina/2/cosa/2)]
=2sinacosa+2(sina)^2*[(sina/2)^2/(sina/2*cosa/2)]
=2sinacosa+2(sina)^2[(1-cosa)/sina)]
=2sinacosa+2sina(1-cosa)
=2sina.