Express this logarithm in terms of a and b

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How do I express $\log_52$ in terms of $a$ and $b$ if:

$\log_62 =a$ and $\log_53 =b$

I've tried:

  • Converting the $a$ and $b$ equations to fractions, and substituting $\log2$ and $\log5$ with $a\log6$ and $(\log3)/b$ respectively, but I ended up with the same equation after simplifying things out.

  • I've also tried to convert the $2$ in $\log_52$ into fractions and go from there, but I went up going in circles and never got anywhere.

How would I solve this question?

2

There are 2 best solutions below

0
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Using change of base (change to base 5):

$$ \log_6 2 = \frac{\log_5 2}{\log_5 6} = \frac{\log_5 2}{\log_5 3 + \log_5 2}$$

$$ a = \frac{\log_5 2}{b + \log_5 2}$$

$$ ab = (1-a)\log_5 2$$

$$ \log_5 2 = ?$$

I trust you can finish the rest

4
On

$a=\log_62=\dfrac{\log2}{\log2+\log3}$ as $\log(ab)=\log a+\log b$ when all the logarithms are defined.

Now express $\log2$ in terms of $\log3$

Again, $b=\log_53=\dfrac{\log3}{\log5}\iff\log5=\dfrac{\log3}b$

Now $\log_52=\dfrac{\log2}{\log5}=?$


Alternatively,

$$\log_53=b\implies3=5^b$$

$$\log_62=a\implies2=6^a=(5^b)^a2^a\iff2^{1-a}=5^{ab}$$

Take log wrt base $5$ $$(1-a)\log_52=ab$$