vector{BA}+vector{AX}=vector{BX}
vector{AX}=(2/9)vector{AC}
vector{AC}=vector{AB}+vector{BC} ( по правилу треугольника)
значит
vector{AX}==(2/9)(vector{AB}+vector{BC})
тогда
vector{BX}=vector{BA}+vector{AX}=vector{BA}+(2/9)(vector{AB}+vector{BC})=vector{BA}-(2/9)(vector{BA}+(2/9)vector{BC})=
=(7/9)(vector{BA}+(2/9) vector{BC})