∵AB是圆O的直径,∴∠AEB=∠AED+∠CEF=90°
∵PC⊥AB,∴∠P+∠PBC=90°
∵∠AED=∠PBC(同弧所对圆周角相等)
∴∠CEF=∠P
∵∠ECF=∠PCD
∴△PCD∽△ECF
∴PC/EC=CD/CF
∴PC*CF=EC*CD
∵AB是圆O的直径,∴∠AEB=∠AED+∠CEF=90°
∵PC⊥AB,∴∠P+∠PBC=90°
∵∠AED=∠PBC(同弧所对圆周角相等)
∴∠CEF=∠P
∵∠ECF=∠PCD
∴△PCD∽△ECF
∴PC/EC=CD/CF
∴PC*CF=EC*CD