# Homework Help: Find the derivative

1. Jun 20, 2010

### lude1

1. The problem statement, all variables and given/known data
Find f' of ln[((x2+1).5) / x(2x3-1)2]

2. Relevant equations

3. The attempt at a solution
I know the derivative of ln(x) is 1/x... however I don't know how to start this particular problem. I think there is a easier way to solve this without trying to solve the derivative of [((x2+1).5) / x(2x3-1)2]..?

2. Jun 20, 2010

You need to use the chain rule. And I don't think there is an "easy" way to do this (i.e. a shortcut). You just need to do a lot of bookkeeping.

Also although you are correct to say that d/dx [ln(x)] = 1/x, that does not really help since the argument of the log function is not 'x' it is a function of x. So we say that if 'u' is a function of x, then d/dx [ln(u)] = 1/u *du/dx.

That is the chain rule applied to the natural log function.

3. Jun 20, 2010

### HallsofIvy

Use the laws of logarithms:

$$\ln\left[\frac{(x^2+ 1)^{.5}}{x(2x^3-1)^2}\right]= .5 ln(x^2+ 1)- ln(x)- 2ln(2x^3- 1)$$

That's a little easier to differentiate.

4. Jun 20, 2010

### Staff: Mentor

Before differentiating, use the properties of logs so that instead of the log of a quotient, you're working with a difference of logs. You will need to use the chain rule when you actually start differentiating.

5. Jun 20, 2010

### HallsofIvy

Wow, three responses in three minutes.

But, this time, I got in before Mark44!!!

6. Jun 20, 2010

### lude1

Thanks! :)

7. Jun 20, 2010

### Staff: Mentor

I'm having a slow day

8. Jun 20, 2010

Muh hahaha! But not before me! Now somebody answer my Incomplete Gamma function question!

9. Jun 21, 2010

### Susanne217

HallsoftIvy,

If this was the star wars univers you would be yoda and the rest of us younglings!

10. Jun 21, 2010

### HallsofIvy

How did you find out I was short and wrinkly?