已知三角形ABC中,角BAC=45度,以AB,AC为边在三角形ABC外作等腰三角形ABD和三角形ACE,AB=AD,AC
3个回答

过点A作AM⊥BE于M,AN⊥CD于N

∵∠BAD=60,AB=AD

∴等边△ABD

∴∠ABD=∠ADB=60

∵∠BAE=∠BAC+∠CAE,∠DAC=∠BAC+∠BAD,∠BAD=∠CAE

∴∠BAE=∠DAC

∵AC=AE

∴△BAE≌△DAC (SAS)

∴∠ABE=∠ADC,BE=CD,S△BAE=S△DAC

∠DFE=∠FBD+∠FDB

=∠ABD+∠ABE+∠FDB

=∠ABD+∠ADC+∠FDB

=∠ABD+∠ADB=120

∵AM⊥BE,AN⊥CD

∴S△BAE=BE×AM/2,S△DAC=CD×AN/2

∴AM=AN

∴AF平分∠DFE

∴∠AFE=∠DFE/2=60°