If `vec(a),vec(b),vec(c)` and `vec(a),vec(b),vec(c)` are reciprocal system of vectors, then prove that `vec(a)xxvec(b)+vec(b)xxvec(c)+vec(c)xxvec(a)=(vec(a)+vec(b)+vec(c))/([vec(a)vec(b)vec(c)])` .