La suma dels angles d'un triangle

A = (-4, 3)
B = (3, 8)
a = Segment(A, B)
C = (9, 1)
b = Segment(B, C)
c = Segment(A, C)
α = Angle(B, C, A)
β = Angle(C, A, B)
γ = Angle(A, B, C)