Hey sorry I'm having a bit of mind blank today, can anyone give me a hint to get me started with this

CheersIf A, B and C are the angles of a triangle prove that

$\displaystyle

\cos \begin{pmatrix} \frac{B-C}{2} \end{pmatrix} - \sin \begin{pmatrix} \frac{A}{2} \end{pmatrix} = 2 \sin \begin{pmatrix} \frac{B}{2} \end{pmatrix} \sin \begin{pmatrix} \frac{C}{2} \end{pmatrix}

$

Simon