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Homework Help: AM GM HM inequality

  1. Oct 28, 2014 #1
    1. The problem statement, all variables and given/known data
    If a+b+c=0 then ( (b-c)/a + (c-a)/b + (a-b)/c )( a/(b-c) + b/(c-a) + c/(a-b) ) is equal to:

    Ans: 9

    2. Relevant equations
    Equality holds when all numbers are equal.

    3. The attempt at a solution
    I tried using AM>=GM.
    ( (b-c)/a + (c-a)/b + (a-b)/c + a/(b-c) + b/(c-a) + c/(a-b) )/6 >= 1

    The AM side doesn't seem to simplify. All the numbers on the GM side cancel leaving me with one. I know I need to convert it into some form of a+b+c=0 but can't figure out how.
  2. jcsd
  3. Oct 29, 2014 #2

    Ray Vickson

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    Science Advisor
    Homework Helper

    The AM/GM inequality is irrelevant in this problem, and does not even apply. The reason is that if a+b+c=0 and at least one of a,b,c is nonzero, then at least one of them is positive and at least one is negative. The AM/GM inequality applies only if all the cited numbers are of the same sign.

    Anyway, put c = -a-b and grind it through (and yes, indeed, it is lengthy!).
  4. Oct 29, 2014 #3
    Oh ok!
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