In the picture, 'A, B, C, D' are points on a circle centred at '0 .' The lines 'A C' and BD are, extended to meet at 'P'. The line 'A D' and 'B C' intersect at 'Q'. Prove that the angle which the small arc AB makes.at 0 is the sum of the angles it makes at 'P' and 'Q'.
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