(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

hi,

I'm working on constructible things again and in one of the proofs our prof threw out this identity and I just don't know where it came from:

[tex]1+e^{\frac{2\pi}{7}i}+e^{\frac{4\pi}{7}i}+e^{\frac{6\pi}{7}i}+e^{\frac{8\pi}{7}i}+e^{\frac{10\pi}{7}i}+e^{\frac{12\pi}{7}i}=\frac{e^(\frac{12\pi}{7}i)^7-1}{e^{\frac{12\pi}{7}i}-1}[/tex]

HOW did he get that?

Edit: I can't tell if the final term is supposed to be 2pi of 12 pi. I dunno.

2. Relevant equations

3. The attempt at a solution

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# Sum of the e's

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