# Math Help - Functions

1. ## Functions

Find f'(x)
don't know how to slove this

3) f(x) = log9(2^x + 1)

the 9 is subscript

2. Originally Posted by bigton
Find f'(x)
don't know how to slove this

3) f(x) = log9(2^x + 1)

the 9 is subscript
$\frac{d}{dx}\bigg[\log_a\left(f(x)\right)\bigg]=\frac{d}{dx}\bigg[\frac{\ln\left(f(x)\right)}{\ln(a)}\bigg]=\frac{f'(x)}{f(x)\ln(a)}$

3. so what rule is this and do i just sub

4. Originally Posted by theflyingcow
so what rule is this and do i just sub
That used many rules. If you have to take the derivative of something in the exact form of $y=\log_{a}f(x)$, then yes you can substitute. But this isn't worth memorizing, I promise you.

5. so subing into the variables into the equation is the answer

6. For this problem, yes.