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IF $x , y , z$ are arbitary positive real numbers satisfying the equation

$ 4xy + 6yz + 8xz = 9$

Find the maximum value of the product $xyz$.

I dont know from where to begin .

3 variables and one equation.

How I can achieve this?

  • 1
    This is the same sort of problem as the o$n$e in [this other question](http://math.stacke$x$change.com/questions/117000/find-the-maximum-possible-value-of-the-equation) of yours. The Wikipedia article on [Lagrange multipliers](http://en.wikipedia.org/wiki/Lagrange_multiplier) shows you how to solve these; it has some worked out examples.2012-03-06

3 Answers 3

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Arithmetic mean is $\ge$ Geometric mean, i.e. ${{4xy + 6yz + 8xz}\over3} \ge {{(4xy\cdot 6yz\cdot 8xz)}}^{1/3}.$

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    When you use AM-GM, we have $\text{AM}=\text{GM}$ iff terms that are averaged are all equal. So put $4xy=6yz=8xz=3$, solve.2012-03-13
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If we want to use the AGM inequality without loss we have to "symmetrize" the three variables. Therefore we replace $x$, $y$, $z$ by x':=4x,\quad y':=3y,\quad z':=6z\ . The AGM inequality then says that \root 3\of {x'^2y'^2z'^2}\leq {x'y' + y'z' + z' x'\over 3}=4xy + 6 yz+8 zx=9\ , i.e., x'y'z'\leq 27, with equality iff x'=y'=z'=3. It follows that xyz = {x'y'z'\over 72}\leq{3\over 8} with equality iff $x={3\over4}$, $y=1$, $z={1\over2}$.

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    @vikiiii: I wrote $x'=\alpha x$, $\ldots$, and determined $\alpha$, $\beta$, $\gamma$ such that $4xy={1\over3}x'y'$, $\ldots\ $.2012-03-07
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You can also do it by using weighted arithmetic mean and geometric mean inequality,

\frac{m_1x_1 + m_2 x_2 + m_3 x_3 +...+ m_n x_n}{m_1+m_2+m_3....m_n} > [{x_1^{m_1}x_2^{m_2}x_3^{m_3}x_4^{m_4}....x_n^{m_n}}]^{\frac{1}{m_1m_1m_2m_3.....m_n}}