If cos 65 degree + cos 55 degree = cos θ, find θ in radians

We have to find t (theta) in radians.

cos t = cos 65° + cos 55°

We know that:

cos x + cos y = 2 · cos (x+y) / 2 · cos (x - y) / 2

cos t = 2 · cos (65°+55°) / 2 · cos (65°-55°) / 2

cos t = 2 · cos 60°· cos 5°

cos t = 2 · 1/2 · cos 5°

cos t = cos 5°

t = 5° = π/36 radians