If tan θ/2 = 1-a1+atan ϕ/2 ,then prove that cos ϕ = cos θ- a1- acosθ .

If $\tan θ / 2 = \sqrt{\frac{1 - a}{1 + a}} \tan ϕ / 2, t h e n p r o v e t h a t \cos ϕ = \frac{\cos θ - a}{1 - a \cos θ}$ .

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