Dalam segitiga ABC diketahui b=8cm, c=5cm dan a= 7. Cos A =

Mapel : Matematika
Kelas : X SMA
Bab : Trigonometri

Pembahasan :
Cos A = (b² + c² - a²)/2bc
Cos A = (8² + 5² - 7²)/80
Cos A = (64 + 25 - 49)/80
Cos A = 40/80
Cos A = 1/2

Aturan cosinus

a^2 = b^2 + c^2 - 2bc . cos A
7^2 = 8^2 + 5^2 - 2.8.5.cosA
49 = 64 + 25 -80cosA
80cosA = 64 + 25 - 49
80cosA = 40
cosA = 40/80
cos A = 1/2