Maths Olympiad Prep

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, 2013

Algebra Difficulty 5.9 AIME, harder Prove it Slovenia

Positive real numbers xx and yy satisfy
2013log3x=ylog52013andlog12x+log12y>0. 2013^{\log_3 x} = y^{\log_5 2013} \quad \text{and} \quad \log_{\frac{1}{2}} x + \log_{\frac{1}{2}} y > 0.
Which of the numbers xx and yy is greater?

Solution

Taking logarithms on both sides of the equation and taking into account that logab=bloga\log a^b = b \log a and logab=logbloga\log_a b = \frac{\log b}{\log a}, we get
logxlog2013log3=logylog2013log5, \frac{\log x \log 2013}{\log 3} = \frac{\log y \log 2013}{\log 5},
or
logy=log5log3logx=log35logx. \log y = \frac{\log 5}{\log 3} \log x = \log_3 5 \log x.
From here we can conclude that logy\log y and logx\log x have the same sign, so xx and yy are either both less than 11, both equal to 11 or both greater than 11.

On the other hand the inequality can be rewritten as log12(xy)>0\log_{\frac{1}{2}}(xy) > 0 and since 12<1\frac{1}{2} < 1 we have xy<1xy < 1.

Both together imply that xx and yy are less than 11, so logx\log x and logy\log y are negative. But log35>1\log_3 5 > 1 implies that logy=log35logx<logx\log y = \log_3 5 \log x < \log x, or y<xy < x.

So, xx is greater.

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