Question

已知向量a(1,2,3),b(-2,-4,-6),IcI=根号14,若(a+b)·c=7,则a与c的夹角为 A.30° B.60° C.120° D.150°

要详细过程

Answer 1

a+b=(-1,-2,-3)=-a,
|a|=√14=|c|,
ac=-7,
∴cos<a,c>=-1/2,
选C.