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.