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Inequality problem

  1. Jun 10, 2013 #1
    1. The problem statement, all variables and given/known data
    Let ##a,b,c## be positive real numbers with sum 3. Prove that
    [tex]\sqrt{a}+\sqrt{b}+\sqrt{c} \geq ab+bc+ca[/tex]

    2. Relevant equations
    AM-GM inequality


    3. The attempt at a solution
    I don't really know how to start with. We are given ##a+b+c=3##.
    Also, ##2(ab+bc+ca)=(a+b+c)^2-(a^2+b^2+c^2)## but this doesn't seem helpful here and I don't see how can I apply the AM-GM inequality here. :(
     
  2. jcsd
  3. Jun 10, 2013 #2
    I think the general statement of the inequality is ##\forall x_i\in\mathbb{R}_+\cup\{0\} \, \frac{x_1+x_2+\cdots+x_n}{n}\geq\sqrt[n]{x_1x_2\cdots x_n}##.
    I had to think about this, but...let's use your hint.

    Evaluate ##2x-x^2(3-x^2)##. What does that imply about positive x?
    Multiply both sides of the statement you need to prove. See where that gets you. :wink:
     
  4. Jun 11, 2013 #3
    Sorry for the late reply but I figured this out later. :smile:

    The original inequality (to be proved) can be re-written as
    [tex]a^2+b^2+c^2+2(\sqrt{a}+\sqrt{b}+\sqrt{c}) \geq 9[/tex]
    This inequality can be easily proved by AM-GM inequality.
    [tex]a^2+\sqrt{a}+\sqrt{a} \geq 3a[/tex]
    Hence proved.
     
  5. Jun 11, 2013 #4
    I didn't think of that.

    ##\forall x\geq0, \, 2x−x^2(3−x^2)=(x−1)^2x(x+2)\geq0 \implies 2x \geq x^2(3−x^2)##.

    If we multiply both sides of the original inequality by 2, we get ##2\sqrt{a}+2\sqrt{b}+2\sqrt{c}\geq 2ab+2bc+2ca = a(b+c)+b(a+c)+c(a+b) = a(3-a)+b(3-b)+c(3-c)##. The proof thus follows as an example of the first inequality by considering the case ##x=\sqrt{\alpha}, \, \alpha\in\{a, b, c\}##.
     
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