a=(1,2),b=(-2,-4), a+b = (-1,-2) = -a5/2 = (a+b)•c= - a•c = |a| |c| cos = √5 * √5 cos=> cos = 1/2=> a与c的夹角为 60°
a=(1,2),b=(-2,-4), a+b = (-1,-2) = -a5/2 = (a+b)•c= - a•c = -|a| |c| cos = -√5 * √5 cos=> cos =- 1/2=> a与c的夹角为 120°729707767他漏了负号
