For the complex equation $\displaystyle z^4 = cosx + isinx$, prove that the sum of the four solutions is always zero, no matter what size $\displaystyle x$ is.

This is what I've done so far.

$\displaystyle Z = (cis(x + 360k))^1^/^4

= cis(x/4 + 90k)$

Can someone please teach me how to do the rest? Thanks in advance