# Math Help - One angle of triangle

1. ## One angle of triangle

I'm not sure what formula is needed to answer the attached question. Maybe the half angle formulae?

2. Hello Stuck Man
Originally Posted by Stuck Man
I'm not sure what formula is needed to answer the attached question. Maybe the half angle formulae?
$\sin A = \sin(\pi-A)$
$=\sin(B+C)$

$=\sin B\cos C + \cos B \sin C$
Can you see it OK from here?