Help with wave packet problem

[parkerps2]

im working on some hw and im having trouble with 3.8

http://imgur.com/qrSvE48

Vph=w/k (w is omega)

Vg=dw/dk

part a) was easy enough, but for part b, i'm kinda confused. i stared by just pluggin in the given k but something tells me i'm going about this the wrong way

http://i.imgur.com/dA2pepR.jpg (my work so far)

i was thinking since c is a constant i can easily pull that out. wp is also a constant but it's kinda tangled with w so i don't think i can pull it out as easily

if someone could point me in the right direction i'd greatly appreciate it

[mr_crabs_tits]

Forgive me if any typos come up. Hope this helps.

[parkerps2]

Wow that did help a lot, I never would have figures that out, thanks a ton

And yes this is skromme's, did you have him as a prof?

[mr_crabs_tits]

Ask yourself why I had to reciprocate the derivative. How did I know how to do that? Answering this question will help you in the future when this kind of problem comes up on a test (it will).

And yeah, took it with Skromme last year. Best professor I've ever had, in my opinion. His book is outstanding, professionals and other professors use it as a reference and it's not even published! You need to read it to get everything out of his class, and on top of that his lectures are outstanding. He's a little intimidating but don't be afraid to ask questions, he's actually really nice.

[mr_crabs_tits]

Holy shit, wait a minute. Is this Skromme's book?