The resultant of two vectors $\vec{A}$ and $\vec{B}$ is perpendicular to $\vec{A}$ and its magnitude

Question

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 $\qquad$?\

Professor PC Thomas Classes · Answer

(150) : $R=\frac{B}{2}$\\ $\begin{aligned} B \cos \alpha & =R \Rightarrow B \cos \alpha=\frac{B}{2} 	ext { or } \cos \alpha=\frac{1}{2} \ \Rightarrow \quad \alpha & =60^{\circ}\end{aligned}$\ $\Rightarrow \alpha=60^{\circ}$\ So, $	heta=\alpha+90=150^{\circ}$