Precalculus (6th Edition)

Published by Pearson
ISBN 10: 013421742X
ISBN 13: 978-0-13421-742-0

Chapter 1 - Equations and Inequalities - 1.4 Quadratic Equations - 1.4 Exercises - Page 121: 29


$\color{blue}{\left\{-9i, 9i\right\}}$

Work Step by Step

RECALL: If $x^2=a$, then taking the square root of both sides gives $x = \pm \sqrt{a}$. Take the square root of both sides of the given equation to obtain: $\sqrt{x^2}=\pm \sqrt{-81} \\x =\pm \sqrt{81(-1)} \\x =\pm 9\sqrt{-1}$ Since $\sqrt{-1}=i$, then the solution is: $x=\pm 9i$ Thus, the solution set is $\color{blue}{\left\{-9i, 9i\right\}}$.
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