we are to showa=(1/2) closed loop integral over [rx dl]

I suppose this can be done formally from the alternative form of Stokes' theorem that can be obtained by replacing the vector field in curl theorem byVxCwhereCis a constant vector

The identity is :

surface int [(daxgrad) xV]=closed loop integral over [dlxV]

The RHS matches.But how to show that LHS leads to the required value?