a)vtIJ=vtIA+vtAJ=-2vtAB+2/5vtAC
b)vtIG=vtIA+vtAG=-2vtAB+2/3vtAM=-2vtAB+1/3(vtAB+vtAC)=-5/3vtAB+1/3vtAC=5/6vtIJ
=>I;J;G thẳng hàng
c)vtI'J'=vtI'A+vtAJ'=-2/3vtAB+2/5vtAC
vtI'O=vtI'A+vtAO=-2/3vtAB+1/2vtAM=-2/3vtAB+1/4(vtAB+vtAC)=-5/12vtAB+1/4vtAC
=>vtI'O=5/8vtIJ'
=>I;O;J' thẳng hàng