When you're proving something to be a cyclic quad, say ABCD, and you know that ACB=ADB, is the statement you make still "angles subtended in same segment are equal"?
If the endpoints of an interval AB subtend equal angles at two points C and D on the same side of the interval, then the endpoints of the interval and the two points are concyclic.
Thus A, B, C, D lie on a circle, so ABCD is a cyclic quadrilateral.
(Note: That's the formal reason. You may be able to get away with writing less in the HSC)