If sin (θ + 18°) = cos 60° (0 < θ

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sin (θ + 18°) = cos 60°
= cos (90° – 30°) = sin 30°
⇒ θ + 18° = 30°
⇒ θ = 30° – 18° = 12°

∴ cos5θ = cos 60° =	1
	2

	1
	√2