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Homework Help: Angles of a Triangle

  1. Feb 22, 2008 #1
    1. The problem statement, all variables and given/known data
    I need to prove the following equation:
    cos^2 A + cos^2 B + cos^2 C + 2*cosA*cosB*cosC = 1
    where A,B,C are the angles of a triangle

    2. Relevant equations
    A+B+C = 180

    3. The attempt at a solution
    I substituted the sum of A,B,C into the equation, and now I have something like
    .... + cos^2(180-A-C)+..... so on, but I don't know what to do next.

  2. jcsd
  3. Feb 22, 2008 #2

    Gib Z

    User Avatar
    Homework Helper

    Welcome to Physicsforums ER901!

    How about using the Cosine rule to express the Cosines as functions of the sides? Theres some messy algebra but it should work out fine.
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