Question

costheta = (1- tan^2(theta/2))/ (1 + tan^2(theta/2))

Answer 1

You want to find theta?

Answer 2

One thing you can do think about the pytagorean identities first off

Answer 3

s^2 + c^2 = 1

divide each side by cosine we get tan^2 + 1 = sec^2