Maths Olympiad Prep

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, 2016

Algebra Difficulty 4.6 AIME Prove it Slovenia

Find all real numbers xx and yy that solve the system of equations
log3x2+log2y3=1, \log_3 x^2 + \log_2 y^3 = 1,
log9x4+log4y9=2. \log_9 x^4 + \log_4 y^9 = 2.

Solution

Denote a=log3x2a = \log_3 x^2 and b=log2y3b = \log_2 y^3. Then
log9x4=log3x4log39=2log3x22=aandlog4y9=log2y9log24=3log2y32=32b. \log_9 x^4 = \frac{\log_3 x^4}{\log_3 9} = \frac{2 \log_3 x^2}{2} = a \quad \text{and} \quad \log_4 y^9 = \frac{\log_2 y^9}{\log_2 4} = \frac{3 \log_2 y^3}{2} = \frac{3}{2}b.
Inserting this into the initial equations we get a+b=1a+b=1 and a+32b=2a+\frac{3}{2}b=2. Subtracting the first equation from the second yields 12b=1\frac{1}{2}b=1 or b=2b=2. From the first equation we get a=1a=-1. From log3x2=1\log_3 x^2 = -1 it follows that x2=13x^2 = \frac{1}{3} or x=±13x = \pm\frac{1}{\sqrt{3}}, and from log2y3=2\log_2 y^3 = 2 we have y3=4y^3 = 4 or y=43y = \sqrt[3]{4}.

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