Straightforward differentiation, but think I have wrong sign somewhere

[phyzmatix]

1. Homework Statement , attempt at a solution

Please see attached. I'm actually busy with a physics problem, but solving it requires that I complete this part correctly. It's straightforward differentiation, but I think I made an error with my signs somewhere and I can't for the life of me find where...

Any pointers? (I believe this is going to be one of those *doh* situations )
phyz

 

[quantumdude]

There's nothing wrong. The quantity in brackets in the last line is in fact zero. Don't forget:

$$x\cdot x^{-7/2}=x^{1-7/2}=x^{-5/2}$$

 

[phyzmatix]

You see...*DOH!*

Thanks for the help!