OM = OA + AM (1)OM = OB + BM (2)(1)等式两边同乘以2，得2OM = 2OA + 2AM (3)(2)(3)等式相加3OM = OB + BM + 2OA + 2AM = OB + 2OA (BM = 2MA)两边同除以3即得
