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連接AC, ∵BD‖CE,∴∠EBC=∠DBC, ∵∠DBC=∠DAC,∴∠ECB=∠DAC, ∵∠EBC=∠ADC(圓內接四邊形的外角等於內對角), ∴ΔEBC∽ΔCDA, ∴BE/DC=BC/AD, ∴AD*BE=BC*DC.
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