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How would one show that for positive $a,b,c,d$ and $a+b+c+d = 4$ that $ \frac{a}{b} + \frac{b}{c} + \frac{c}{d} + \frac{d}{a} \leq \frac{4}{abcd} $

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    C'mon people. I'm not a 15 year old in the middle of an exam. Give me a constructive hint or better yet, show a complete solution. I also have the Lagrange multiplier solution, but I think it's too inelegant. I'm looking for something more stylish.2012-02-02

3 Answers 3

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Consider

$\left(\frac{a}{b} + \frac{b}{c} + \frac{c}{d} + \frac{d}{a}\right)abcd = a^2cd + b^2ad + c^2ab + d^2bc = ac(ad + bc) + bd(ab + cd)$

Since there is cyclic symmetry, we can assume that $ad + bc \le ab + cd$.

So

$ac(ad + bc) + bd(ab + cd) \le (ac + bd)(ab + cd)$

Now $xy \le \left(\frac{x+y}{2}\right)^2$

and so

$(ac + bd)(ab + cd) \le \left(\frac{ac + bd + ab + cd}{2}\right)^2 = \left(\frac{(a+d)(b+c)}{2}\right)^2$

Applying $xy \le \left(\frac{x+y}{2}\right)^2$ again we get

$\left(\frac{(a+d)(b+c)}{2}\right)^2 \le \left(\frac{\left(\frac{a+b+c+d}{2}\right)^2}{2}\right)^2 = 4$

Thus $\left(\frac{a}{b} + \frac{b}{c} + \frac{c}{d} + \frac{d}{a}\right)abcd \le 4$

and so

$\frac{a}{b} + \frac{b}{c} + \frac{c}{d} + \frac{d}{a} \le \frac{4}{abcd}$


What we have shown is that, for four positive numbers,

$ \left(\frac{a+b+c+d}{4}\right)^4 \ge abcd\frac{\left(\frac{a}{b} + \frac{b}{c} + \frac{c}{d}+ \frac{d}{a}\right)}{4}$

and since $\frac{a}{b} + \frac{b}{c} + \frac{c}{d}+ \frac{d}{a} \ge 4$, this inequality is stronger than $\text{AM} \ge \text{GM}$ for $4$ numbers.

Somewhat surprisingly, we only used $\text{AM} \ge \text{GM}$ (twice) to prove it! And for two numbers, a similar inequality is actually false!

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    I cannot resist to agree with the comments here. This is a really beautiful proof.2012-02-25
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Aryabhata's nice proof can be restated as :

$ (ac+bd)((a+b+c+d)^4 - 64(abcc+bcdd+cdaa+dabb)) \\ =ac(16(ac+bd-ad-bc)^2+(a+b-c-d)^2((a+b+c+d)^2+4(a+b)(c+d))) \\ +bd(16(ac+bd-ab-cd)^2+(b+c-d-a)^2((a+b+c+d)^2+4(b+c)(d+a))) \\ \ge 0$

Therefore, if $a+b+c+d = 4$, $abcc+bcdd+cdaa+dabb \le 4$.

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Let's assume $a,b,c,d>0$. Rewriting your equation gives: $ \begin{eqnarray*} a^2cd+b^2ad+c^2ab+d^2bc\leq 4 \end{eqnarray*} $ Equality is reached, if $a=b=c=d=1$. It's left to show, that this maximal:

Let $b=(2-a)$ with $0 and $c=d=1$. Substituting this, gives $ \begin{eqnarray*} (a-2)(a-1)^2&<&0 \end{eqnarray*} $ which is true for the given range of $a$.

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    you are right. I took it back +1 for yours2012-02-24