We start with the formula for the cosine of a double angle that we met in the last section. Now, if we let then 2θ = α and our formula becomes:We n...

Using a similar process, with the same substitution of θ=α2\displaystyle\theta=\frac{\alpha}{{2}}θ=2α​ (so 2θ = α) we subsitute into the identityWe...

The tangent of a half angle is given by:ProofWe can also write the tangent of a half angle as follows: Proof