# Math Help - derivative of 1/x^(2/5) + 8/x^(5/7)?

1. ## derivative of 1/x^(2/5) + 8/x^(5/7)?

I'm completely confused by this

2. ## Re: derivative of 1/x^(2/5) + 8/x^(5/7)?

Originally Posted by rhcprule3
I'm completely confused by this
Hint: You know that the derivative of $x^n$ is equal to $n \cdot x^{n - 1}$...

-Dan

3. ## Re: derivative of 1/x^(2/5) + 8/x^(5/7)?

yes i know, actually got what i think is the derivative and for the problem im working on, I got f'(1) correct but when I plug in 4 for f'(4) it says its incorrect! Error in the system?

What i got was -2/5(x)^(-3/5) - 5/7(8x)^(-2/7)

For f'(1) I got -6.114 and I got that correct
When I got in f' (4) I got -.4395 and it says its incorrect

4. ## Re: derivative of 1/x^(2/5) + 8/x^(5/7)?

Another hint: f(x) can be written as:

$f(x) = x^{-2/5} + 8x^{-5/7}$

-2/5 -1 is not equal to -3/5. And -5/7 - 1 is not equal to -2/7.