1. ## function

how do I write tan^2(t/2) as a function of cost, where tan^2(t/2)=f(cost)?

2. Try the identity:

$tan^{2}(u)=\frac{1-cos(2u)}{1+cos(2u)}$

3. thanks, I'll try that.

4. so.. I get 1-cost/1+cost, but then I'm rather lost again.. do i multiply both with 1-cost...?

5. It's entirely in terms of cosine. That's it, isn't it?.

Don't you get: $\frac{1-cos(t)}{1+cos(t)}$?.

6. yeah, that's what I meant.. but if the answer's supposed to be a polynomial?