∵D是BC边的中点，DE垂直AB，DF垂直AC∴∠BED=∠DFC=90° BD=CD在RT△BED和RT△DFC中ED=FDBD=DC∴RT△BED≌RT△DFC HL∴∠B=∠C∴AB=AC
∵BD=CD∴S△ABD=S△ACDAB×DE=AC×DF又∵DE=DF∴AB=AC
'''''''''''' 就是这样
