If a , b and c are non-co-planar vectors such that x1a+y1b+z1c=3x2a+4y2b+5z2c then value of x1x2+y1y2+z1z2 is:
The coordinates of the mid-point of the line segment joining two points P(X1, y1, z1) and Q(x2, y2, z2) are: