它的对角线好像垂直证明：作BE⊥AC于E,DF⊥AC于F,则：AB^2=BE^2+AE^2CD^2=DF^2+CF^2AD^2=DF^2+AF^2BC^2=BE^2+CE^2因为AB^2+CD^2=AD^2+BC^2,所以,AE^2+CF^2=AF^2+CE^2所以,E,F必须重合即：B,E,F,D共线所以,BD⊥AC...