Why is tangent's period 180 degrees?

In order for tan x = 0, sin x must be = 0 
sin x = 0 at 0 degrees and 180 degrees 
so, tan x = 0 at 0, pi 
as for the period being pi, you need to have tan x be negative and positive 
so remember the acronym All students take calculus ( ASTC) for where they are positive. 
tan x is negative in Quadrant II and positive in Quadrant I, since it just repeats after that, the period therefore must be Pi.

21 is the answer the answer is 21