Answer
$-i$
Work Step by Step
Factor out as many $i^4$ as possible.
$i^{-13}=(i^4)^{-3}*i^{-1}$
Know that $i^4=1$. Substitute, and simplify.
$(i^4)^{-3}*i^{-1}=1^{-3}*i^{-1}=\frac{1}{i}=-i$
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