I'm trying to get the 4 solutions for this:

$\displaystyle w^4 = -8 +(8*3^\frac{1}{2})i $

In cis form and after applying de Moivre's theorem I get:

$\displaystyle 2cis( \frac{-\pi}{12} +\frac{1}{2}k\pi)$

Where k is varying integer.

I've checked several times and I can't see where I'm wrong.

Help appreciated!