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I humbly ask for help in the following problem.

If \begin{equation} A+B+C=180 \end{equation} Then prove \begin{equation} \cos A+\cos B+\cos C=1+4\sin(A/2)\sin(B/2)\sin(C/2) \end{equation} How would I begin the problem I mean I think $\cos C $ can be $\cos(180-A+B)$. But I am unsure what to do next.

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    tried to [google](http://sg.answers.yahoo.com/question/index?qid=20091218205518AAtsWzf) it?2012-07-30
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    Oh I see its on yahoo answer, so would I delete this question?2012-07-30
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    Underneath the tags--that is, `trigonometry` and `trigonometric-identities`--there's a delete option. That said, you don't *have* to delete it.2012-07-30
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    Hmm I will keep the question for posterity.2012-07-30
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    Then for posterity I think we should link to [Y! Answers](http://sg.answers.yahoo.com/question/index?qid=20091218205518AAtsWzf).2012-07-30

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