It 2pi/3 because in an isosceles triangle angle between two equal side is 120 . In this case as A and B sides are equal so it is 2pi/3 above of all if you add A vector n B vector you will get 2root3kcap .
Mitali answered this
Okay.. i got it.. I just realised i was doing one silly mistake of minus.. Thanks for helping..☻☺☻