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Question
Find the angle θ, when 0° < θ < 90°.
cos θ = cos2 30° − sin2 30°
Sum
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Solution
cos θ = cos2 30° − sin2 30°, 0° < θ < 90°
cos2 − sin2 = cos2
cos2 30° − sin2 30° = cos(2 × 30°) = cos 60°
cos θ = cos 60°
cos 60° = `1/2`
cos θ = `1/2`
θ = 60°
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