如图5-83.已知DE.BF分别平分∠ADC和∠ABC.∠1=∠2.∠ADC=∠ABC.则AB∥CD.试写出推理过程. 图5-83 解:∵DE.BF分别平分∠ADC和∠ABC . ∴∠l=——青夏教育精英家教网—

摘要：如图5-83.已知DE.BF分别平分∠ADC和∠ABC.∠1=∠2.∠ADC=∠ABC.则AB∥CD.试写出推理过程. 图5-83 解:∵DE.BF分别平分∠ADC和∠ABC . ∴∠l=∠ABC.∠CDE=∠ADC. ∵∠ADC=∠ABC.∴∠1=∠CDE 又∵∠1=∠2.∴∠CDE=∠a. ∴AB∥DC(内错角相等.两直线平行)

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19、如图所示，已知DE，EF是△ABC的两条中位线．求证：四边形BFED是平行四边形．