Answer
i
Work Step by Step
In Section 7.7, we derived that $i^{4}=1$. Therefore, we can use this fact to write the equation in terms of $i^{4}$ and find the desired power of i.
$i^{21}=i^{20}\times i^{1}=(i^{4})^{5}\times i=1^{5}\times i=1\times i=i$
