ABC is a triangle inscribed in a circle with centre O. Let α = ∠ BAC, where 45° < α < 90° Let β = ∠BOC. Which one of the following is correct?
a. cosβ = (1 - tan2α)/(1 + tan2α)
b. cosβ = (1 + tan2α)/(1 - tan2α)
c. cosβ = 2tanα/(1 + tan2α)
d. sinβ = 2sin2α