BC is divided into four equal parts by P, Q and R. The resultant of AB→ and 3 AC→ is
AR→
4AR→
4AP→
PQ→
AC→+CB→=AB→; AC→+4CR→=AB→
AC→+4(AR→-AC→)=AB→;4AR→-3AC→=AB→
∴AB→+3AC→=4AR→