Diketahui a-B = 60° dan sin a sin B = 2/5. Nilai cos (a+B) = Tolong jawab cepat ya. Terimakasih.

Bab Trigonometri
Matematika SMA Kelas X

cos (a - b) = cos 60°
cos a . cos b + sin a . sin b = 1/2
cos a . cos b + 2/5 = 1/2
cos a . cos b = 1/2 - 2/5
cos a . cos = 5/10 - 4/10
cos a . cos b = 1/10

cos (a + b)
= cos a . cos b - sin a . sin b
= 1/10 - 2/5
= 1/10 - 4/10
= -3/10