∴∠ 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