OD=OA+AD=OA+1/2AB=OA+1/2(OB-OA)=1/2(OA+OB)=1/2(a+b)
OE=OA+AE=OA+1/3AB=1/3OB+2/3OA=2/3a+1/3b或
OE=OA+AE=OA+2/3AB=2/3OB+1/3OA=1/3a+2/3
OD=OA+AD=OA+1/2AB=OA+1/2(OB-OA)=1/2(OA+OB)=1/2(a+b)
OE=OA+AE=OA+1/3AB=1/3OB+2/3OA=2/3a+1/3b或
OE=OA+AE=OA+2/3AB=2/3OB+1/3OA=1/3a+2/3