Relationships? Hmm...

I found cyclic quadrilaterals
Quite similar to the Day 12 quadrilateral.
The are both quadrilaterals (obviously), the cyclic quadrilateral has opposite sides angles up to the same thing (\(180^{\circ}\))
 and the new quadrilateral has opposite sides sum up to the same thing
and they are all related to circles.
Hmm...interesting.

Wait are they actually the same thing?!