Ipotesi\!:\ a \perp c;\ b \perp c\
Tesi\!:\ a \parallel b

1\ -\ Sia\ (per\ assurdo)\ a \not\parallel b\ -\ 
Negazione\ della\ tesi

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

3\ -\ \triangle AOB\ ha\ 2\ angoli\ retti\ -\ 
Per\ (2)\ e\ (Ip.)\ -\ Impossibile

4\- \ a \parallel b\ -\ (1)\ è\ falsa\ per\ (3)\
Q.\ E.\ D.

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A_1 = (2.28, 4.88)
B_1 = (2.28, -0.88)
c = Line(A_1, B_1)
C = (-0.52, 3.68)
a = OrthogonalLine(C, c)
A = Intersect(c, a)
D = (-0.46, 2.42)
b = OrthogonalLine(D, c)
B = Intersect(c, b)
O = (7.46, 3.06)
a' = Ray(O, A)
b' = Ray(O, B)