i+j+k,a1i+b1j+c1k,a2i+b2j+c2k,a3i+b3j+c3k are the position vectors of the points A,B,C,D respectively. ‌
2
3
(i+j+k) is the position vector of the centroid of the triangular face BCD of the tetrahedron ABCD . If αi+βj+γk is the position vector of the centroid of the tetrahedron, then 2α+β+γ=