11. Dalam segitiga ABC diketahui

Sudut BAC = 30° => sudut A = 30°
Sudut ABC = sudut B
AC/(sin B) = BC/(sin A)
2/(sin B) = (3/2)/(sin 30°)
2/(sin B) = (3/2)/(1/2)
2/(sin B) = 3
Sin B = 2/3 = depan/miring
Samping = √(3^2 - 2^2) = √(9 - 4) = √5
Cos ABC = cos B = samping/miring = √5/3

Mapel : Matematika
Kelas : X SMA
Bab : Trigonometri

Pembahasan :
Aturan Sinus

AC/Sin B = BC/Sin A
2/Sin B = (3/2)/Sin 30°
2(1/2) = 3/2 Sin B
3/2 Sin B = 1
Sin B = 2/3

Identitas Trigonometri
Sin²B + Cos²B = 1
Cos B = √[1 - (Sin B)²]
Cos B = √[1 - (2/3)²]
Cos B = √[1 - 4/9]
Cos B = √[5/9]
Cos B = 1/3√5