Untuk sudut lancip, diketahui : Cos α = 3/15 dan sin β = 1/2√2 Tentukan A. Cos (α + β) B. Cos (α - β)

Alpha = a
Beta = b

cos a = 3/15 = 1/5
sin a = √(1 - cos² a) = (2/5)√6

sin b = cos b = (1/2)√2

cos (a + b)
= cos a cos b - sin a sin b
= (1/5)(1/2)√2 - (2/5)(√6)(1/2)√2
= (1/10)√2 - (2/5)√3

cos (a - b)
= cos a cos b + sin a sin b
= (1/10)√2 + (2/5)√3