# How to solve 3^x = 2187 by using Casio fx-85GT plus

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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Regards,

• Take logs of both sides, any base you like. That will give you $x\log 3 =\log {2187}$ and you can now just divide. – lulu Sep 15 '18 at 15:26
• Thanks lulu, it worked! – Bateman Sep 15 '18 at 16:53
• 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 – Michael Hoppe Sep 15 '18 at 16:54
• @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. – Robert Howard Nov 30 '18 at 16:22

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.