∵DA切⊙O于A、DBC是⊙O的割线,　∴由切割线定理,有：AD^2＝DB×DC.
∵PD＝AD,　∴PD^2＝DB×DC,　∴PD/DB＝DC/PD,又∠PDC＝∠BDP,
∴△PDC∽△BDP,　∴PD/DB＝CP/PB,　∴DB×PC＝PD×PB,而PD＝AD,
∴DB×PC＝AD×PB.