A Level

MATHEMATICS · Subjective · A Level · Mathematics · Trigonometry · Trigonometry

Problem

Show that

\cos \alpha + \cos \beta + \cos \gamma + \cos(\alpha + \beta + \gamma) = 4 \cos \frac{\alpha + \beta}{2} \cdot \cos \frac{\beta + \gamma}{2} \cdot \cos \frac{\gamma + \alpha}{2}