Answer
Please see below.
Work Step by Step
Let $\theta$ be any angle in standard position and let $P=(x,y)$ be a point on the terminal side of $\theta $. Then $\tan \theta $ is defined as $\tan \theta = \frac{y}{x}$. So this definition makes sense if $x \neq 0$. However, for $\theta =90^{\circ }$, any point on the terminal side of this angle has $x=0$. Thus, $\tan 90^{\circ }$ cannot be defined.
