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Homework Help: Prove this inequality

  1. Aug 16, 2013 #1


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    Gold Member

    1. The problem statement, all variables and given/known data
    In a triangle ABC, prove by vector method cos2A+cos2B+cos2C≥ -3/2.

    2. Relevant equations

    3. The attempt at a solution
    I can change the LHS of the inequality to the form
    (cos2A i + cos2B j + cos2C k).(i+j+k)
  2. jcsd
  3. Aug 16, 2013 #2


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    Homework Helper

    hi utkarshakash! :smile:

    hint: 2A + 2B + 2C = 360° …

    so draw a circle! :wink:
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