Prove this trignometric function cos(theta/2) = 1/(sqrt(1+t^2)

Question

Prove/show that

cos (θ/2) = (1/√(1+t²))

Answer 1

cos (θ/2) = (1/√(1+t²))

θ=30°; t=6

cos15=1/(1+36)^.5

.965=..164????????????

Need mot imfo.

Depends on θ.