check the picture below, notice the red asymptotic lines.
now, recall that
[tex]\bf tan(\theta)=\cfrac{sin(\theta)}{cos(\theta)}
\\\\\\
tan\left( \frac{\pi }{2} \right)=\cfrac{sin\left( \frac{\pi }{2} \right)}{cos\left( \frac{\pi }{2} \right)}\implies \stackrel{und efined}{tan\left( \frac{\pi }{2} \right)=\cfrac{1}{0}}
\\\\\\
tan\left( \frac{3\pi }{2} \right)=\cfrac{sin\left( \frac{3\pi }{2} \right)}{cos\left( \frac{3\pi }{2} \right)}\implies \stackrel{und efined}{tan\left( \frac{\pi }{2} \right)=\cfrac{-1}{0}}[/tex]
