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How could you prove using only the AM GM inequality:

$\frac{a}{b} + \frac{b}{c} + \frac{c}{a} \ge a + b + c$

Where $abc = 1$ and $a,b,c$ are positive real numbers?

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2 Answers 2

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Hint: $$\frac{a}{b}+\frac{a}{b}+\frac{b}{c}\ge 3\sqrt[3]{\frac{a^2b}{b^2c}}=3\sqrt[3]{a^3}=3a$$ $$\frac{b}{c}+\frac{b}{c}+\frac{c}{a}\ge 3\sqrt[3]{\frac{b^2c}{c^2a}}=3\sqrt[3]{b^3}=3b$$ $$\frac{c}{a}+\frac{c}{a}+\frac{a}{b}\ge 3\sqrt[3]{\frac{c^2a}{a^2b}}=3\sqrt[3]{c^3}=3c$$

Add all: what do you get?

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Because $abc=1$ we can consider some real numbers $x,y,z$ such that $a=\frac{x}{y}$, $b=\frac{y}{z}$, $c=\frac{z}{x}$. Computing this into your equation, we get:

$$\frac{\frac{x}{y}}{\frac{y}{z}}+\frac{\frac{y}{z}}{\frac{z}{x}}+\frac{\frac{z}{x}}{\frac{x}{y}}\geq\frac{x}{y}+\frac{y}{z}+\frac{z}{x}$$ which is equivalent to $$\frac{xz}{y^2}+\frac{yx}{z^2}+\frac{zy}{x^2}\geq\frac{x}{y}+\frac{y}{z}+\frac{z}{x}$$ and multiplying by $(xyz)^2$ both sides, we get $$(xy)^3+(yz)^3+(zx)^3\geq x^3z^2y+y^3x^2z+z^3y^2x$$

Now write $xy=m$, $yz=n$, $zx=p$ and we get $$m^3+n^3+p^3\geq p^2m+n^2m+m^2p$$ which is a known inequality.

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