∵向量|a|=3,|b|=4,且向量ab夹角为120°,
∴
⑴ a·b=|a||b|cos120°=3 x 4 x (-0.5) = -6
⑵ (a+b)² = a² + 2a·b + b² = 9 +(-12) +16 =13
⑶ |a-b| = √(a-b)² =√(a² - 2a·b + b² ) =√37