# I really need an answer-Derive trig identity

Dainy
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(sin2x+sin4x)/(cos2x+cos4x)=tan3x

Homework Helper
You can use these to prove that trig identity:
$$\cos \alpha + \cos \beta = 2 \cos \frac{\alpha + \beta}{2} \cos \frac{\alpha - \beta}{2}$$
$$\cos \alpha - \cos \beta = - 2 \sin \frac{\alpha + \beta}{2} \sin \frac{\alpha - \beta}{2}$$
$$\sin \alpha + \sin \beta = 2 \sin \frac{\alpha + \beta}{2} \cos \frac{\alpha - \beta}{2}$$
$$\sin \alpha - \sin \beta = 2 \cos \frac{\alpha + \beta}{2} \sin \frac{\alpha - \beta}{2}$$
Choose 2 of the above equations, and use them.
Can you go from there?
Viet Dao,