Rumus trigonometri yang sering digunakan:
\(\sin A + \sin B = 2 \sin \frac 12 (A + B) \cos \frac 12 (A - B)\)
\(\sin A - \sin B = 2 \cos \frac 12 (A + B) \sin \frac 12 (A - B)\)
\(\cos A + \cos B = 2 \cos \frac 12 (A + B) \cos \frac 12 (A - B)\)
\(\cos A - \cos B = -2 \sin \frac 12 (A + B) \sin \frac 12 (A - B)\)
\(\tan A + \tan B = \tan (A + B) \:.\: [1 - \tan A \tan B ]\)
\(\tan A - \tan B = \tan (A - B) \:.\: [1 + \tan A \tan B ]\)