Using the fact that z=e^ibeta(adsbygoogle = window.adsbygoogle || []).push({});

and the identity 1 + z + z^2...+z^n=1-z^(n+1)/1-z

Help me derive

1 + cosbeta +cos2beta + ... +cosnbeta= 1/2 + sin((2n+1)beta/2)/2sin(beta/2) where beta is between 0 and 2pi

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# Help me derive Lagrange's Trigonometric Identity

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