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∵PA切⊙O於A,∴∠PAB=∠ACB. ∵PA切⊙O於A,∴AD⊥AP、PB⊥BD,∴A、D、B、P共圓,∴∠PAB=∠PDB. 由∠PAB=∠ACB、∠PAB=∠PDB,得:∠ACB=∠PDB,∴AC‖PD.
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