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**techmath** The phase velocity for waves in a determined material is given by: v= $\displaystyle v=C_1 + C_2 \lambda$

Both Cs are constants. $\displaystyle \lambda$ is wavelength. Determine the group velocity $\displaystyle v_g$!

My calculation:

$\displaystyle \omega=v \cdot k$

$\displaystyle k=\frac{2 \pi}{\lambda}$

$\displaystyle v_g=\frac{d \omega}{dk} = \frac{d(vk)}{dk}= (C_1 +C_2 \lambda)'k + (C_1 +C_2 \lambda)k' = C_2k + (C_1 + C_2 \lambda)(-\frac{2 \pi}{\lambda^{2}})$

This is unfortunately incorrect answer!

This is physics, but purely a mathematical difficulty.