Answer
-1
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.
$i^{42}=i^{40}\times i^{2}=(i^{4})^{10}\times i^{2}=1^{10}\times -1=1\times -1=-1$
Here, we are using the fact that $i^{2}=-1$.
