If \(\cos \theta=\frac{a \cos \phi+b}{a + b \cos\phi}\), then \(\tan\frac{\theta}{2}\) is equal to
1
\(\sqrt{\left(\frac{a-b}{a+b}\right)}\tan\frac{\phi}{2}\)
2
\(\sqrt{\left(\frac{a+b}{a-b}\right)}\cos\frac{\phi}{2}\)
3
\(\sqrt{\left(\frac{a-b}{a+b}\right)}\sin\frac{\phi}{2}\)
4
None of these