## Trigonometry (10th Edition)

If $a$ and $b$ are real numbers, then any number of the form $a+bi$ is a complex number. Since $a=0$ and $b=-7$ and both $a$ and $b$ are real numbers, $-7i$ is a complex number. For a complex number $a+bi$, if $a=0$ and $b\ne0$, then the complex number is said to be a pure imaginary number. Since $-7i$ fits this criteria, it is a pure imaginary number. Since pure imaginary numbers are a subset of the set of non-real complex numbers, $-7i$ is a non-real complex number as well.