Diketahui (A+B) = π/3 dan Sin A + Sin B = 1/4. Nilai Cos (A-B) .....

Strateginya gunakan rumus berikut :
Sin A + Sin B = 2 sin1/2(A + B) . cos1/2(A - B)
1/4 = 2 sin 1/2(π/3).cos1/2(A - B)
1/4 = 2.sin π/6.cos1/2(A-B)
Cos1/2(A-B) = 1/4 : 1 = 1/4
Selanjutnya, gunakan rumus sudut paruh :
Cos1/2(A - B) = √((1 + cos(A - B)/2)
1/4 = √((1+cos(A - B))/2)
(1/4)² = 1/2(1+cos(A - B))
1/16 = 1/2 + 1/2 cos(A - B)
Cos(A - B) = (1/16-1/2)/(1/2) = -7/16/(1/2) = -14/16 = -7/8
Semoga membantu, sebelum itu mohon di cek dulu y.