17) We leave it to the reader as an exercise to verify
that the position vector of the point R which divides
the line segment PQ externally in the ratio
m : n 
PR
 i e QR
 
 
 
 
 
 
nm
 is given by
OR
uuur  =
mb
na
m
−n
−
r
r
Remark If R is the midpoint of PQ , then m = n