Jika di ketauhi cos 20° = a maka nilai cos 70° cos 50° =

Cos 70.cos 50 = 1/2(cos(70+50)+cos(70-50)) = 1/2 cos 120+1/2 cos 20 = 1/2(-1/2)+1/2a = -1/4+1/2a = (-2+4a)/8 = (4a - 2)/8 (pembilang dan penyebut dibagi 2)
1/4(2a - 1)