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2014-06-07T20:27:02+07:00
α - β = π/4 , cosα . cosβ = 1/5 .
Cos (α - β) = cosαcosβ + sinαsinβ
Cos π/4     =      1/5 + SinαSinβ
     1/2 √2  = 1/5 + SinαSinβ
SinαSinβ = 1/5 - 1/2 √2
  atau langsung saja
Cos (α - β) = Cos π/4 = 1/2 √2
2 5 2