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4. If a = 3 + b, then ${a}^{3}-{b}^{3}-9abis:$

1. 9

2. 27

3. 81

4. 18

10. In the figure chord AB and CD when produced meet at point P. If $\angle AOC=\theta ,\angle BOD=\alpha $ then prove that $\angle APC=\frac{\theta -\alpha}{2}$.

(O is the centre of the circle)

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