vector{AB}+vector{BC}=vector{AC}
⇒ vector{AC} =vector{a}+vector{b}
По правилу треугольника
vector{AB} + vector{BM} =vector{AM}
Так как
vector{АМ}=(1/2)*vector{AC} ⇒
vector{AМ} =(1/2)*vector{a}+(1/2)*vector{b}
значит
vector{а} + vector{BM} =(1/2)*vector{a}+(1/2)*vector{b}
⇒
vector{BM} =(1/2)*vector{a}+(1/2)*vector{b}- vector{а}
⇒
vector{BM} =(1/2)*vector{b}-(1/2)*vector{a}