5 2  Vector joining two points
If P1(x1, y1, z1) and P2(x2, y2, z2) are any two points, then the vector joining P1 and P2
is the vector 
1 2
P P
uuuur  (Fig 10 15) Joining the points P1 and P2 with the origin
O, and applying triangle law, from the triangle
OP1P2, we have
1
1 2
OP
uuur+P P
uuuur  =
OP