Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$-1$
Using $i^2=-1$ and the laws of exponents, the given expression, $i^{42} ,$ is equivalent to \begin{array}{l}\require{cancel} \left( i^2 \right)^{21} \\\\= \left( -1 \right)^{21} \\\\= -1 \end{array}