выразить через вектор А и В вектор КВ
vector{AF}=vector{AD}+vector{DC}+vector{CF} ⇒ vector{b}=2vector{a}+vector{DC}-vector{a} ⇒ vector{DC}= vector{b}-vector{a} vector{KB}=vector{KD}+vector{DA}+vector{AB}=-(1/2)(vector{b}-vector{a})-2vector{a}+(vector{b}-vector{a})=(1/2)*vector{b}-(5/2)*vector{a}