Ipotesi\!:\ A\^BC \cong D\^CA\
Tesi\!:\ AB \parallel CD

1\ -\ Sia\ (per\ assurdo)\ AB \not\parallel CD\ - 
Negazione\ della\ tesi

2\ -\ \exists O\ tale\ che\ O \in a \wedge O \in c\ -\ 
Per\ (1)

3\ -\ A\^BC < D\^CA\ - Teorema\ angolo\ 
esterno

4\ -\ AB \parallel CD\ -\ (1)\ è\ falsa\ perchè\ (3)\ 
contraddice\ (Ip.)\
Q.\ E.\ D.

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A = (5.94, 2.3)
B = (4.06, 2.3)
a = Line(A, B)
C = (5.24, 3.9)
b = Line(B, C)
α = Angle(A, B, C)
D = Rotate(B, (-α), C)
β = Angle(D, C, B)
c = Line(C, D)
E = (10.86, -1.14)
F = (14.9, 1.86)
G = Point(c)
d = CircleArc(E, F, G)
O = Intersect(d, a, 1)