∫AD and CD are divided equally ∠OAB and ∠OCB,

∴∠OAD+∠OCD= 12×40 =20，

∴∠adc=∠o+∠oad+∠ocd=80+20 = 100

⑵∫∠OAD+∠OCD =∠ADC-∠O

∠OAB+∠OCB=∠ABC-∠O

∫AB and CD are divided equally ∠OAB and ∠OCB respectively.

∴∠OAB+∠OCB=2(∠OAD+∠OCD)

∴2(∠ADC-∠O)=∠ABC-∠O

∴2∠ADC-2∠O=∠ABC-∠O

∴∠O+∠ABC=2∠ADC