1373. Special quadrilaterals

    A circumscriptible (or tangential) quadrilateral has the following property: its opposite sides add up to the same length (the semiperimeter).

    An inscriptible (or cyclic) quadrilateral has the following property: its opposite angles add up to the same width (a streight angle).

    Nina Guindilla proved both properties...

    Dear Teacher:

a + c = e + f + g + h

b + d = e + f + g + h

2α + 2γ = 360°
α + γ = 180°

and as well

 β + δ = 180°