Trigonometry

Always ready to help. You can tell us the general idea to see if you're on the right track. We won't necessarily tell you the right answer, but we can tell you where is wrong, or what is flawed thinking. [That's what educators are for.]

Hadriel

Thanks, but I finally managed to solve it.

I went with P(k-1) and P(k) instead of P(k) and P(k+1) so I wouldn't have to deal with cos(k+1)θ.