Answer: Option A
:
A
From the triangle law of vector addition, it is clear that the vectors R, A and B must be representedby the three sides of an equilateral triangle because the magnitudes of the three vectors are equal. Hencethe angle $\alpha$ between vectors R and A is ${60}^{\circ }$. Alternatively, the value of $\alpha$is given by
$\tan \alpha =\frac{B\sin \theta }{A+B\cos \theta }=\frac{\sin \theta }{1+\cos \theta }(\because A=B)$
$=\frac{\sin {120}^{\circ }}{1+\cos {120}^{\circ }}=1.732$
Which gives $\alpha ={60}^{\circ }.$
Hence the correct choice is (a)