Chapter 8 - Radical Functions - 8.2 Simplifying, Adding, and Subtracting Radicals - 8.2 Exercises - Page 633: 9

$\sqrt{-9}$ is not a real number because there is no real number that when squared gives $-9$.

Note that: $\sqrt{9} = 3$ because $3^2=9$ $\sqrt{16}=4$ because $4^2=16$ The given radical is NOT A REAL NUMBER because there is no real number that when squared gives $-9$.