Diketahui sin α = 4/5 , cos β = 7/25 , α dan β sudut lancip. Nilai cos ( α - β ) =.....??

sin a = 4/5
cos a = 3/5

cos b = 7/25
sin b = 24/25

cos (a-b) = sin a × sin b + cos a × cos b
cos (a-b) = 4/5 × 24/25 + 3/5 × 7/25
cos (a-b) = 96/125 + 21/125 = 117/125

sin a = depan/miring
= 4/5

cos a = samping/miring
samping = √(5²-4²)
= √(25-16)
= √9
= 3
cos a = 3/5

sin b = depan/miring
= 7/25

cos b = samping/miring
samping = √(25²-7²)
= √(625/49)
= √576
= 24
cos b = 24/25

cos (a -b) = cos a cos b + sin a sin b
= (3/5)(24/25) + (4/5)(7/25)
= (72/125) +(28/125)
= 100/125
= 4/5