简单分析一下，详情如图所示
OG=ON+NG=(OB+OC)/2+NM/3=(OB+OC)/2+(NA+AM)/3=(OB+OC)/2+(OA-ON+AM)/3= (OB+OC)/2+(OA-(OB+OC)/2-OA/2)/3=OA/6+OB/3+OC/3
