The magnitude of the resultant of two equal vectors is equal to the magnitude of either vector and the angle between these two vectors is 120$°$. The angle between the resultant and either vector will be 

From the triangle law of vector addition, it is clear that the vectors R, A and B must be$$\begin{gathered} represented by the three sides of an equilateral triangle because the magnitudes of the three vectors are equal. Hence the angle a, between vectors R and A is 60°. Alternatively, the value of a is given by \\ \tan \alpha =\frac{B\sin \theta }{A+Bcs\theta }=\frac{\sin \theta }{1+\cos \theta } \left(Therefore, A=B\right) \\ =\frac{\sin 120°}{1+\cos 120°}=1.732 \\ which gives \alpha =60° \end{gathered}$$