Two l vectors having equal magnitude , have a resultant equal to either of the two. The angle between them is

Let two vectors are A and B, inclined at an angle θ. Resultant of the two vectors A and B,|R|=|A|2+|B|2+2|A||B|cosθLet |A|=|B|=aAccording to the question, R = aFrom Eq. (i), we geta=a2+a2+2aacosθ⇒ a2=a2+a2+2a2cosθ⇒ 2a2cosθ=-a2⇒ cosθ=-1/2⇒ θ=120°