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89

Part of 1975 Chisinau City MO

Problems(1)

Chisinau MO p89 1975 R1 VIII circle criterion as a closed line

Source:

3/16/2021
A closed line on a plane is such that any quadrangle inscribed in it has the sum of opposite angles equal to 180o180^o. Prove that this line is a circle.
Cycliccirclegeometry