[b]An illustration of the angle properties to be found in a cyclic quadrilateral.[br][/b][br][br]Circle Theorems [url]http://www.geogebratube.org/student/c89/m1462/yryy[/url]

[b]This is a nice theorem to prove algebraically.[/b][br][br]As a start to your proof you could mark the centre of the circle and [br]draw in the radii to the vertices of the quadrilateral.[br][br]A nice alternative is to draw the diagonals and use another popular circle theorem[br]that you can see illustrated here [url]http://www.geogebratube.org/student/m1462[/url]