Answer
$ 4i$
Work Step by Step
Based on the given conditions, we have:
$[\sqrt 2(cos\frac{5\pi}{8}+i\ sin\frac{5\pi}{8})]^4\\ =4(cos\frac{4*5\pi}{8}+i\ sin\frac{4*5\pi}{8})\\
=4(cos\frac{5\pi}{2}+i\ sin\frac{5\pi}{2})=4i$
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