Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

$$y = \log_7(e^{-x}\cos\pi x)$$

I got: $$y' = \frac {-\sin\pi xe^{-x} - \cos\pi x e^{-x}}{e^{-x}\cos\pi x\ln(7)}$$

Is that correct?

share|improve this question

1 Answer 1

In general, the derivative of $\log_a y$, where $y$ is a function of $x$, is

$$\frac{y'}{y \ln a},$$

which I believe you know.

You dropped a $\pi$ when you took the derivative of $\cos (\pi x)$. The derivative is $-\pi \sin(\pi x)$ by the chain rule. Other than that, looks good.

share|improve this answer
    
Where did I drop the $\pi$? –  dsta Oct 21 '12 at 2:21
    
Ok, thank you! I appreciate it. –  dsta Oct 21 '12 at 2:24
    
@dsta No problem, glad to help. It's nice that you had already made an attempt and showed what you got. You're not just trying to get someone to do your homework, but to actually learn! –  Graphth Oct 21 '12 at 2:40

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.