Hi Juriemagic,
Not both are true but i expect that is because you made a typo.
Think about the cyclic nature of the tan function. It repeats every 180 degrees
If you consider it works on the unit circle, that should help.
\(tan\theta = tan(\theta\pm180k) \qquad \text{where k is an integer}\\ so\\ tan\theta = tan (\theta -180)=tan(-(-\theta+180))\\ also\\ tan\theta = tan(\theta+180)=tan(-(-\theta-180))\\ \\~\\ HOWEVER\\ tan \theta =tan(\theta-180)\ne tan(-(\theta+180))\\ but\\ tan \theta = -tan(-(\theta+180))\\\)
Here are some of those graphs
https://www.desmos.com/calculator/foqjyque7o
I hope that helps :)