The problem: prove that
I am not sure how you "prove" this.
Now that's going to confuse a lot of people, Prove It!
More generally, tan and cotan, sin and cos, sec and csc "swap" complementary angles. That is, if and are the two non-right angles in a right triangle, then [tex]tan(\theta)= cot(\phi), , [tex], and . And, of course, those two angles add to (did I get it right, Prove It?).