Yi Zheng ∠AFH=∠ABC

And < PFC = < afh,

When PC is tangent to circle O, ∠PCF=∠ABC.

∴∠PFC=∠PCF

When △PFC satisfies ∴ when PC=PF, PC is tangent to ⊙ O.

Da 2 = de * df, then △DAF∽△DEA.

∴∠DAF=∠DEA=∠DCA

∴ arc DA= arc DC

At this time, d is the midpoint of the bad arc AC.