The vectors OA, OB, OC, let OA = λ 1ob+λ 2oc. Then λ 1+λ 2 = 1 is equivalent to the three-point * * line of A, B and C, which is proved as follows:

λ 1 = 1-λ 2, if substituted, there is OA = λ1ob+(1-λ1) oc, that is, OA-oc = λ 1 (ob-oc).