When is $\tan(a+b)$ undefined? For what values of $a$ and $b$ is $\tan(a+b)$ undefined? What is the relationship between $a$ and $b$ when it is undefined? 
What about for $\tan(a-b)$?

In degrees:
$\tan(90)$ is undefined.
Therefore is $a+b=90$ or $a-b=90$ create undefined solutions for whatever $a$ and $b$ that fit this equation.
The tan of any odd multiple of $90$ such as $90, 270, 450,$ etc. is also undefined.  
