Interpretation: apply the formula sin α = [2 tan (a/2)]/[1+(tan α/2) 2].
cosα=[ 1-tan(α/2)^2]/[ 1+tan(a/2)^2]
tana=[2tan(a/2)]/[ 1-tan(a/2)^2]
Replacing tg(α/2) with sinα, cosα and tgα is called universal substitution.
Deduction: sina=2sin(a/2)cos(a/2)
=[2sin(a/2)cos(a/2)]/[sin(a/2)^2+cos(a/2)^2]
=[2tan(a/2)]/[ 1+(tanα/2)^2]
Cosa is the same as tana.