My question is, how do I show that speed is equal to group velocity? More information at https://imgur.com/a/m6FwNaG
v_g = dw/dk
The Attempt at a Solution
Part a is substitution, part b uses v_g = dw/dk, part c is multiplication by h-bar, but I am stuck at part d. We are dealing with a relativistic energy equation, but the question says "the speed v" as if that has some meaning. What am I missing? I have a feeling I am overlooking something incredibly simple.