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East China Normal University Edition Ninth Grade Mathematics
CO = AO

∴∠ 1=∠A

CO = BO

∴∠2=∠B

The straight line CO intersects the circle o at point d.

∠BOD=∠2+∠B=2∠2

∠AOD=∠ 1+∠A=2∠ 1

∠AOB=∠AOD-∠BOD

=2∠2-2∠ 1

=2(∠2-∠ 1)

=2∠ACB