Maths Olympiad Prep

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Algebra Difficulty 5.6 AIME, harder Prove it United States

Problem:
Given that xx, yy, and zz are positive real numbers such that
xlog2(yz)=2834,ylog2(zx)=2936,andzlog2(xy)=25310,x^{\log_{2}(yz)} = 2^{8} \cdot 3^{4}, \quad y^{\log_{2}(zx)} = 2^{9} \cdot 3^{6}, \quad \text{and} \quad z^{\log_{2}(xy)} = 2^{5} \cdot 3^{10},
compute the smallest possible value of xyzxyz.

Solution

Solution:
Let k=log23k = \log_{2} 3 for brevity. Taking the base-22 log of each equation gives
(log2x)(log2y+log2z)=8+4k,(\log_{2} x)(\log_{2} y + \log_{2} z) = 8 + 4k,
(log2y)(log2z+log2x)=9+6k,(\log_{2} y)(\log_{2} z + \log_{2} x) = 9 + 6k,
(log2z)(log2x+log2y)=5+10k.(\log_{2} z)(\log_{2} x + \log_{2} y) = 5 + 10k.
Adding the first two equations and subtracting the third yields 2log2xlog2y=122 \log_{2} x \log_{2} y = 12, so log2xlog2y=6\log_{2} x \log_{2} y = 6. Similarly, we get
log2xlog2y=6,\log_{2} x \log_{2} y = 6,
log2ylog2z=3+6k,\log_{2} y \log_{2} z = 3 + 6k,
log2zlog2x=2+4k.\log_{2} z \log_{2} x = 2 + 4k.
Multiplying the first two equations and dividing by the third yields (log2y)2=9(\log_{2} y)^{2} = 9, so log2y=±3\log_{2} y = \pm 3. Then, the first and last equations tell us log2x=±2\log_{2} x = \pm 2 and log2z=±(1+2k)\log_{2} z = \pm (1 + 2k), with all signs matching. Thus
log2x+log2y+log2z=±(3+2+(1+2k))=±(6+2k),\log_{2} x + \log_{2} y + \log_{2} z = \pm (3 + 2 + (1 + 2k)) = \pm (6 + 2k),
so
xyz=2±(6+2k)=2632or2632.x y z = 2^{\pm (6 + 2k)} = 2^{6} \cdot 3^{2} \quad \text{or} \quad 2^{-6} \cdot 3^{-2}.
Clearly, the smallest solution is 2632=[1576]2^{-6} \cdot 3^{-2} = \left[\frac{1}{576}\right].

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.