∵ tana/2=(sina/2)/(cosa/2)
=2 [(sina/2)× (cosa/2)]/2cos2(a/2)
= sina/2(cosa/2)2 ∵ cosa = 2cos2(a/2)-1
∴tana/2= sina/2(cosa/2)2 = sina/ 1+ cosa
tana/2=（sina/2）/(cosa/2)=(2sina/2 *cosa/2)/[2(cosa/2)^2]=sina /1+cosa
tan(a/2)
=sin(a/2)/cos(a/2)
=2sin(a/2)cos(a/2)/2cos(a/2)^2
=sin(a)/(cos(a/2)^2+sin(a/2)^2+cos(a/2)^2-sin(a/2)^2)
=sin(a)/(1+cos(a))
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