Trigonometry (10th Edition)

Since $\sqrt (24)$ is an irrational number and irrational numbers are a subset of the set of real numbers, $\sqrt (24)$ can be labelled as a real number. For a complex number $a+bi$, if $b=0$, then $a+bi=a$, which is a real number. Thus, the set of real numbers is a subset of the set of complex numbers. Since $\sqrt (24)$ is a real number, it can also be labelled as a complex number.