- 30 POIN - #NOSPAM Materi : Rumus Segitiga dengan Trigonometri Diketahui segitiga ABC dengan AB = 7 cm ; AC = 5 cm ; < BCA = 30° ; maka cos B = .... ?

AB/sin C = AC/sin B
7/sin 30 = 5/sin B
7 sin B = 5 sin 30
sin B = 2,5/7
sin B = 5/14

sisi depan = 5
sisi miring = 14
sisi samping = 3 akar 19

maka
cos B = 3akar19/14

AB = 7 cm
AC = 5 cm
∠C = 30

Aturan sinus
AB / sin30 = AC / sinB
AB. SinB = AC. Sin30 7.SinB = 5 x 1/2
SinB = 5/2 x 1/7
SinB = 5/14

Sin²A + Cos²B = 1
Cos²B = 1 - Sin²B
CosB = √(1 - Sin²B)
CosB = √(1 - (5/14)²)
CosB = √(1 - 25/196)
CosB = √(196/196 - 25/196)
CosB = √(171/196)
CosB = 3√19 / 14