Expressions for the group velocity

[bon]

Homework Statement

Okay so I've been deriving various expressions for the group velocity starting with

g=dw/dk

then g=v+kdv/dk

also g=v-lambda dv/dlambda (*)

then finally g=c/u(1+lambda/u (du/dlambda))

Ok so finally I am asked to use equn (*) to derive the following expression

g=v[1-1/(1+(v/lambda')(d lambda'/dv))

where lambda' is the wavlength in vacuum..


not sure how to do it..any help would be great thanks

Homework Equations

The Attempt at a Solution

 

[bon]

Any ideas?