如图,在三角形ABC中,AB=AC,角BAC=90°,BD垂直MN,CE垂直MN

∵BD⊥MN,∴∠ABD+∠BAD=90°
∵∠BAC=90°,∴∠BAD+∠CAE=90°
∴∠ABD=∠CAE
∵AB=AC,∠ADB=∠CEA=90°
∴△ABD≌△CAE
∴AD=CE,AE=BD
∴DE=AD+AE=BD+CE