由题意可知,等腰Rt△ABC三线合一,H为AB中点,AH=BH=CH,则HD=BD=1/2CH,
易得三角形CDH相似于三角形BDF,则DF=1/2BF=1,勾股定理知BD=根号5,可得BH=2倍根号5
则AC=BC=根号2倍BH=2倍根号10