The trig functions are cyclic, because they repeat every 360 degrees or 2(pi) radians. Cotangent is cosine/sine, so cot is zero when cosine is zero, i.e., when x=90 degrees. But the cosine is zero when x=270, 450, 630, etc. this is best expressed as 90+180n degrees, where n is a positive or negative integer. These are the zeroes of the function f(x)=cotx.