 \vec{ s }\;=\; \vec{ f_1 }\;+\; \vec{ f_2 }   

| \vec{ s }|\;=\; \sqrt{  \vec{ f_1 }²\;+\; \vec{ f_2 }²\;+\;| \vec{ f_1 } |.| \vec{ f_2 } |.cosα   }   

A = (-0.9030519134467829, 0.3926763037109272)
B = (0.7421437757194077, 4.321978629292315)
f_1 = Vector(A, B)
D = (6.172886827335959, 0.48851294579827814)
f_2 = Vector(A, D)
α = Angle(D, A, B)
a = Line(B, f_2)
b = Line(D, f_1)
C = Intersect(a, b)
s = Vector(A, C)
c = Segment(C, B)
d = Segment(C, D)
β = Angle(A, B, C)