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B Minimum value

  1. Feb 28, 2017 #1
    if ## a,b,c,d,e ## are positive real numbers, minimum value of [tex](a+b+c+d+e)( \frac{1}{a} +\frac{1}{b} +\frac{1}{c} +\frac{1}{d} +\frac{1}{e} )[/tex]
    (A) 25
    (B) 5
    (C) 125
    (D) cannot be determined

    My approach :
    expanding the expression , i get
    [tex]5+a( \frac{1}{b} +\frac{1}{c} +\frac{1}{d} +\frac{1}{e} )+ similar.terms.of.b,c,d,e[/tex]

    I cant find any ways to make those expressions vanish .....and some hit and trial gives me ans as 25
    but i also cant find any way to make them 20 .... :( ..
    or is the answer cannot be determined ?
    pls help ....i dont have the answer ...
     
  2. jcsd
  3. Feb 28, 2017 #2
    Have you heard of the power mean inequalities? The hint is to make use of the fact that the arithmetic mean is always greater than or equal to the harmonic mean.
    (and yes I'm not giving any explicit expressions here because I want you look them up and try to understand them - the Art of Problem Solving wiki is a good place to start)
     
  4. Mar 1, 2017 #3
    thanx a lot for the reference Sir ..... So, the direct application of power mean inequality (by putting k1=1 and k2= -1 )gives me the expression is greater than or equal to 25 .... and i hope that is the answer :)
     
    Last edited: Mar 1, 2017
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