若向量a=(1,2),b=(1,-1),则2a+b与b的夹角为A0B2/兀C3／兀D兀

Question

若向量a=(1,2),b=(1,-1),则2a+b与b的夹角为
A0 B2/兀 C3／兀 D兀

Answer 1

2a+b=(3,3),b=(1,-1),
I2a+bI=3√2,IbI=√2
∴c0sθ=0
∴θ=90°
2a+b与b的夹角为90°.