Hello, logan6!

Prove that quadrilateral ABCD is cyclic

if diagonal and diagonalCode:D * * C o- - - - - - -o * / * * \ * * / * \ * / * * \ */ * * \* A o - - - - - - - - - - - o B

Then right triangle is inscribed in semicircle

Then right triangle is inscribed in semicircle

Hence, quadrilateral is inscribed in semicircle

Therefore: .quadrilateral is cyclic.