Hi sunrise: Many trig exercises at the introductory level require rewriting expressions in terms of sines and cosines only. (Even when exercises may not require this, it's still often helpful to think about.)
Hence, students of trig ought to consider completing such a rewrite, often. Cheers :cool:
EDITS: Write f(x)=cot(x) in terms of sine and cosine
Change the sign of the input, and determine how the output changes
Simplify your result to show that changing the sign of cotangent's input causes its output to also change sign
That demonstration proves that cotangent is an odd function