OA=NM/2+BA/2
OB=NM/2+AB/2
OC=MN/2+DC/2
OD=MN/2+CD/2
prin adunare se obtine:
OA+OB+OC+OD=(MN/2+NM/2)+(MN/2+NM/2)+(BA/2+AB/2)+(DC/2+CD/2)
in membrul drept intre fiecare pereche de paranteze (puse in mod voit) avem sume de vectori de module egale si sensuri opuse si prin urmare in fiecare paranteza e un vector nul.
prin urmare avem:
OA+OB+OC+OD=0