Maths Olympiad Prep

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Algebra Difficulty 6.1 National Olympiad Prove it Philippines

Problem:

Suppose a real number x>1x > 1 satisfies

log33(log3x)+log3(log27x)+log27(log33x)=1 \log_{\sqrt[3]{3}}\left(\log_{3} x\right) + \log_{3}\left(\log_{27} x\right) + \log_{27}\left(\log_{\sqrt[3]{3}} x\right) = 1

Compute log3(log3x)\log_{3}\left(\log_{3} x\right).

Solution

Solution:

Let y=log3xy = \log_{3} x. Then x=3yx = 3^{y}.

We compute the inner logarithms:

- log3x=y\log_{3} x = y
- log27x=log3xlog327=y3\log_{27} x = \frac{\log_{3} x}{\log_{3} 27} = \frac{y}{3}
- log33x=log3xlog333=y1/3=3y\log_{\sqrt[3]{3}} x = \frac{\log_{3} x}{\log_{3} \sqrt[3]{3}} = \frac{y}{1/3} = 3y

Now substitute into the equation:

log33(log3x)+log3(log27x)+log27(log33x)=1\log_{\sqrt[3]{3}}(\log_{3} x) + \log_{3}(\log_{27} x) + \log_{27}(\log_{\sqrt[3]{3}} x) = 1

That is:

log33(y)+log3(y3)+log27(3y)=1\log_{\sqrt[3]{3}}(y) + \log_{3}\left(\frac{y}{3}\right) + \log_{27}(3y) = 1

Compute each term:

1. log33(y)=log3ylog333=log3y1/3=3log3y\log_{\sqrt[3]{3}}(y) = \frac{\log_{3} y}{\log_{3} \sqrt[3]{3}} = \frac{\log_{3} y}{1/3} = 3 \log_{3} y

2. log3(y3)=log3ylog33=log3y1\log_{3}\left(\frac{y}{3}\right) = \log_{3} y - \log_{3} 3 = \log_{3} y - 1

3. log27(3y)=log3(3y)log327=log33+log3y3=1+log3y3\log_{27}(3y) = \frac{\log_{3}(3y)}{\log_{3} 27} = \frac{\log_{3} 3 + \log_{3} y}{3} = \frac{1 + \log_{3} y}{3}

Sum:

3log3y+(log3y1)+1+log3y3=13 \log_{3} y + (\log_{3} y - 1) + \frac{1 + \log_{3} y}{3} = 1

Combine like terms:

3log3y+log3y+log3y3=1+1133 \log_{3} y + \log_{3} y + \frac{\log_{3} y}{3} = 1 + 1 - \frac{1}{3}

But let's be careful:

3log3y+log3y=4log3y3 \log_{3} y + \log_{3} y = 4 \log_{3} y

So:

4log3y+log3y31+13=14 \log_{3} y + \frac{\log_{3} y}{3} - 1 + \frac{1}{3} = 1

(4+13)log3y1+13=1\left(4 + \frac{1}{3}\right) \log_{3} y - 1 + \frac{1}{3} = 1

133log3y23=1\frac{13}{3} \log_{3} y - \frac{2}{3} = 1

133log3y=1+23=53\frac{13}{3} \log_{3} y = 1 + \frac{2}{3} = \frac{5}{3}

log3y=513\log_{3} y = \frac{5}{13}

Recall y=log3xy = \log_{3} x, so log3y=log3(log3x)\log_{3} y = \log_{3}(\log_{3} x).

Therefore,

513\boxed{\frac{5}{13}}

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