I need to solve 3^x = 2187 by using Casio fx-85GT plus and I couldn't find anything in its manual.

I know how to do it manually and the answer is "x = 7". But how can I do it using the calculator. Please advise, many thanks!!

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  • 2
    $\begingroup$ Take logs of both sides, any base you like. That will give you $x\log 3 =\log {2187}$ and you can now just divide. $\endgroup$ – lulu Sep 15 '18 at 15:26
  • $\begingroup$ Thanks lulu, it worked! $\endgroup$ – Bateman Sep 15 '18 at 16:53
  • $\begingroup$ An educated guess on $x$ would be $x=7$. Now try to verify that $3^7=2187$. For that purpose type $$3 x^\square 7=$$ on your calc $\endgroup$ – Michael Hoppe Sep 15 '18 at 16:54
  • $\begingroup$ @lulu Could you please convert your comment into an answer to remove this question from the "Unanswered" queue? Seems like you've answered this question pretty thoroughly. $\endgroup$ – Robert Howard Nov 30 '18 at 16:22

As requested in the comments:

Taking $\log$ helps. We see $$x\times \log 3 = \log 2187\implies x= \frac {\log 2187}{\log 3}$$

Here I intend the natural log, but any base would work. Using any sort of calculator one then gets $x=7$.

Note: it is easy to verify that $3^7=2187$ which confirms the result.


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