 \vec{ AB } +  \vec{ AD } =  \vec{ AC }  

A = (1, 1)
B = (3, 4)
u = Vector(A, B)
D = (5, 1)
v = Vector(A, D)
c: u + v
C = Point(c)
a = Segment(B, C)
b = Segment(C, D)