If a quadrilateral ABCD is inscribed in a circle, then the opposite angles of the quadrilateral are supplementary.
I'm not sure how to write this proof. Any help would be greatly appreciated!
This was not intentional.
Join all corners of the quadrilateral to the circle centre "o".
you now have 4 isosceles triangles with the angles at the circumference within each triangle equal.
Therefore 2A+2B+2C+2D =360 degrees.