Bantuin, sama jalannya sekalian

Anggap A = 8 m dan B = 12 m
sudut apit diantara dua vektor A-B (180° – 60°) = 120°
cos 120° = – 1/2

R = √[A² ⁺ ( |-B| )² + 2 A |B| cos 120°]
R = √[8² ⁺ 12² ⁺ 2 (8) (12) (-1/2)]
R = √64 + 144 – 96
R = √208 – 96
R = √112
R = √16 . √7
R = 4 √7