Question

Pada ∆ ABC,jika diketahui panjang sisi b=7cm,sisi c=8cm, dan sdut A = 120° maka panjang sisi a adalah...(cos 120° = -1/2)

Tolong sama caranya yaa

Answer 1

Verified answer

A^2 = b^2 + c^2 - 2bc cos A
a^2 = 7^2 + 8^2 - 2 . 7 . 8 . cos 120°
a^2 = 49 + 64 - 2 . 56 . -1/2
a^2 = 49 + 64 + 56
a = √169
a = 13

Answer 2

Verified answer

Mapel : Matematika
Kelas : X SMA
Bab : Trigonometri

Pembahasan :
Aturan Cosinus

Cos A = (b² + c² - a²)/2bc
Cos 120° = (7² + 8² - a²)/2(7)(8)
-1/2 = (49 + 64 - a²)/112
-1/2 = (113 - a²)/112
-112/2 = 113 - a²
a² = 113 + 112/2
a² = 113 + 56
a² = 169
a² = 13²
a = 13 cm