(2sina-sin2a)/(2sina+sin2a)
=[2sina(1-cosaa)]/[2sina(1+cosa)]
=(1-cosa)/(1+cosa)
=[(cos^2 a/2 +sin^2 a/2)-(cos^2 a/2 -sin^2 a/2)]/[(cos^2 a/2 +sin^2 a/2)+(cos^2 a/2 -sin^2 a/2)]
=(2sin^2 a/2)/(2cos^2 a/2)
=tan^2 a/2