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4 This is homework but I’m really stuck. The question is to prove a fromula which states:
$$1+\cos\theta+\cos2\theta+...+\cos n\theta=\frac{1}{2}+\frac{\sin(n+\frac{1}{2})\theta}{2\sin\frac{\theta}{2}}$$
I want to solve it using complex numbers So I came to this (I missed Re in last one) Can you guys please help me finish this ?
1 Answer
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4 You can refer this :
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This way you can easily do to prove Lagrange’s trigonometric identity:
original answer
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