# Derivative of logarithmic function

If the function $f(x)=\log_{2x}x^2$ is given, what is $f'(4)$?

I tried to use the formula for derivative of logarithm but here the base is $2x$, so it made me confused.

Note that the answer is $1/(18\ln 2)$.

• Are you expecting that people will fix your questions all the time? – Kaster Aug 21 '13 at 23:45
• @Kaster Sorry if I made a mistake but what do you mean by fixing? – guest Aug 21 '13 at 23:47
• I think he's talking about your lack of math formatting. – Ataraxia Aug 21 '13 at 23:47
• @guest I meant your inability to learn how to format math after asking 4 questions. – Kaster Aug 21 '13 at 23:48
• @guest - I've fixed up the question, please look at the formatting so you can learn how to do it yourself next time. – nbubis Aug 21 '13 at 23:49

Note that: $$\log_b a = \frac{\ln a}{\ln b}$$ Now you can use the formula for the derivative of a fraction.