a. Using cos ( + ) cos cos sin sin , or otherwise, show that cos 105 = 2 -

a. Using cos (θ + α) ≡ cos θ cos α − sin θ sin α, or otherwise, show that cos 105° = √2 - √6/4.

b. Hence, or otherwise, show that sec 105° = −√a(1 + √b), where a and b are constants to be found.