if you don't have a Unit Circle, this is a good time to get one, you can find many online.
Check the picture below.
so, let's notice that, -180° has a positive counterpart angle of 180°, they both land on the same spot, thus cot(-180°) is the same as cot(180°).
[tex]\bf cot(-180^o)\implies cot(180^o)\implies \cfrac{cos(180^o)}{sin(180^o)}\implies \cfrac{-1}{0}\implies und efined[/tex]
