Q21. The resultant of two vectors $\vec { A }$ and $\vec { B }$ is perpendicular to $\vec { A }$ and its magnitude is half that of $\vec { B }$. The angle between vectors $\vec { A }$ and $\vec { B }$ is $\_\_\_\_$ ○.
Q21. The resultant of two vectors $\vec { A }$ and $\vec { B }$ is perpendicular to $\vec { A }$ and its magnitude is half that of $\vec { B }$. The angle between vectors $\vec { A }$ and $\vec { B }$ is $\_\_\_\_$ ○.