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

• Oct 2nd 2013, 06:59 PM
rhcprule3
derivative of 1/x^(2/5) + 8/x^(5/7)?
I'm completely confused by this
• Oct 2nd 2013, 08:29 PM
topsquark
Re: derivative of 1/x^(2/5) + 8/x^(5/7)?
Quote:

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
• Oct 3rd 2013, 04:55 AM
rhcprule3
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
• Oct 3rd 2013, 06:08 AM
ebaines
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.