\; \text{Súčet protiľahlých uhlov v tetivovom štvoruholníku je rovný } 180°.

 \text{Postup podľa Euklida} 

1.  \; ∡CAB=∡BDC;∡ACB=∡ADB

2.  \; ∡ADC=∡BAC+∡ACB

\text{ ... pripočítame }\red {∡ABC}

3.  \; ∡ADC +\red {∡ABC}=∡BAC+∡ACB+\red {∡ABC}  \;  \; ...\;  \;=180°

A = (3.5332277289723297, 6.296266808480517)
B = (8.82810939393132, 6.555961277061613)
C = (8.741544571070957, 3.425200183611746)
c = Circle(A, B, C)
D = Point(c)
q1, a, b, c_1, d = Polygon(A, B, C, D)
f = Segment(A, C)
g = Segment(B, D)
α = Angle(C, A, B)
β = Angle(B, D, C)
γ = Angle(A, C, B)
δ = Angle(A, D, B)
ε = Angle(A, B, C)