When angle PAB = 15 degrees it is provable. Construct the diagram .With a compass the circles meet in P
You cannot just say DC, DP and CP are radii of the 2 circles that you have drawn. Can you prove that point P intersects the 2 circles that you have drawn?
Here's the exact same question, answered really well by Archie Meade.
Math Help Forum - Prove
Read post #17 (Second one down). The diagram is pretty much self explanatory.