Answer
$\sqrt {-1}$
Work Step by Step
According to the definition of an imaginary unit on page 734 of the book,
$i=\sqrt {-1}.$
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.