vector{b} = vector{BC} = (-1-5; 3-2) = (-6; 1)
vector{ab} = 3*(-6) + 5*1 = -18 + 5 = -13
|vector{a}| = sqrt(3^2 + 5^2) = sqrt(9 + 25) = sqrt(34)
|vector{b}| = sqrt(37)
теперь найдем угол между векторами
cosphi = vector{ab} / |vector{a}||vector{b}| = -13/35.5
phi = arccos(13/35.5) = 68.52 градусов