2a

α

O = Point({0, 0})
A = Point({2, 1})
AA = Point({4, 2})
a = Vector(A)
aa = Vector(AA)
B = Point({1, 3})
b = Vector(B)
ab: (a * b)
lal: abs(a)
lbl: abs(b)
α: acosd(ab / ((lal * lbl)))
σ = Angle(a, b)