已知向量a,b满足(2a-b)点击（a+3b）=-

已知向量a,b满足(2a-b)点击（a+3b）=-60,a膜等于2,b膜等于4,求a.b夹角（a,b属于向量）

2个回答

(2a-b)点击（a+3b）==2a*a+5a*b-3b*b=2*4+5*2*4*cosa(a ,b)-3*4*4=-60
40cos(a ,b)=-20 cos(a ,b)=-1/2
a.b夹角5π/6