(a) Define the position vector of a point in three–dimensional space.
(b) State the formula for the coordinates of a point that divides internally the line segment joining P(x1,y1,z1) and Q(x2,y2,z2) in the ratio m:n.
(c) Write the particular form of the above formula when the point is the midpoint of PQ.
(d) List any two practical situations in which the internal division of a line segment is required in Mathematics or Physics.