a) Given that 2 sin cos = 1, show that 2 cos +

a) Given that 2 sin² θ − cos θ = 1, show that 2 cos² θ + cosθ − 1 = 0.

b) Hence solve the equation 2 sin²(θ  − 20°) − cos (θ − 20°) − 1 = 0
for 0° ≤ θ ≤ 180°.