# Chapter 7 - Review Exercises - Page 578: 87

$9i$

#### Work Step by Step

Solve $\sqrt{-81}$ As per square root property, we have $\sqrt{81}=\sqrt{9 \cdot 9}=\sqrt{9^2}=9$ Thus, $\sqrt{-81}=9\sqrt {-1}$ Since, $i^2=-1$ and $\sqrt {-1}=i$ Hence, $\sqrt{-81}=9i$

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