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Cos (α - β) = cosαcosβ + sinαsinβ
Cos π/5 = 1/4 + SinαSinβ
1/4√(10- 2√5) = 1/4 + SinαSinβ
SinαSinβ = 1/4√(10-2√5) - 1/4 = 1/4(√(10-2√5) - 1)
atau langsung saja
Cos (α - β) = Cos π/5 = 1/4(√(10 - 2√5)