The solution of the given equation is :
cosθ = 1/2 when θ = π/3 + 2kπ or 5π/3 + 2kπ
cosθ = -1 when θ = π + 2kπ
Since cos2θ + sin2θ = 1, sin2θ = 1 - cos2θ
So, the given equation is equivalent to 2(1 - cos2θ) - cosθ = 1
-2cos2θ - cosθ + 1 = 0
2cos2θ + cosθ - 1 = 0 (multiplying the entire equation by -1)
(2cosθ - 1)(cosθ + 1) = 0 (taking common)
cosθ = 1/2 or cosθ = -1 (on equating it to zero)
cosθ = 1/2 when θ = π/3 + 2kπ or 5π/3 + 2kπ
cosθ = -1 when θ = π + 2kπ
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