p306

in my textbook it writes:

given: $\displaystyle z^4 =1+i$

$\displaystyle z = [\sqrt 2 (cos \frac \pi 4 + i sin\frac \pi 4 )]^ {\frac 1 4}$

where does the $\displaystyle \pm$ go?

i don't understand why it shouldn't be $\displaystyle z = \pm [\sqrt 2 (cos \frac \pi 4 + i sin\frac \pi 4)]^ {\frac 1 4}$