tolong di jawab ya,kalo bisa secepat nya

Aturan Cosinus

BC² = AB² + AC²  - 2 (AB)(AC) cos A
(10√3)² = (20)² +(10)² - 2(20)(10) cos A
300 = 400 + 100 - 400 cos A
300 = 500 - 400 cos A
400 cos A = 500 - 300
400 cos A = 200
cos A = 200/400
cos A = 1/2 = cos 60°
<A = 60°