IF OABC is a tetrahedron where O is the origin and A,B and C have respective positions vectors as `bara,barb and barc` then prove that the circumcentre of the tetrahedron is `((bara)^2(barb times barc)+(barb)^2 (barc times bara)+(barc)^2 (bara times barb))/(2[bara barb barc])`