Answer
$10+i\sqrt 2$
Work Step by Step
$\frac{20+\sqrt -8}{2}=\frac{20+\sqrt (-4\times2)}{2}=\frac{20+2i\sqrt 2}{2}=\frac{2(10+i\sqrt 2)}{2}=10+i\sqrt 2$
We know that $\sqrt -1=i$, because $i^{2}=-1$.
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