the divergence of curl of a vector is always equal to

The curl is a cross product. A cross product of two parallel vectors is zero but the scalar product is zero if the vectors are orthogonal. If you take the vector product of A and B you get C which is orthogonal to A and B, say AxB=C If you calculate now A∗C=A∗(AxB). But C is orthogonal to A which means the scalar product has to be zero.