diketahui segita ABC, AC=21 cm, AB=15 cm, dan sin C =3/7. Nilai cos B adalah..

Bab Trigonometri
Matematika SMA Kelas X

AC/sin B = AB/sin C
21 / sin B = 15 / (3/7)
sin B = 21 x 3/7 : 15
sin B = 3/5 = y/r → Tripel Pitagoras ◎◎◎3, 4, 5
maka, x = 4

cos B = x/r = 4/5