It is known that in △ ABC, ∠ ACB = 90 ° AC = BC, the straight line Mn passes through point C, and ad ⊥ Mn is in D, be ⊥ Mn is in E

It is known that in △ ABC, ∠ ACB = 90 ° AC = BC, the straight line Mn passes through point C, and ad ⊥ Mn is in D, be ⊥ Mn is in E

It is proved that: ① ∵ - ACB = 90 °, be ⊥ CE, ad ⊥ CE, ∵ - BEC = - ACB = - ADC = 90 °, ∵ - ACE + - BCE = 90 °, ∵ BCE + - CBE = 90 °, ∵ ACD = - CBE, in △ ADC and △ CEB, ≌ - ADC = - BEC ≌ ACD = - cbeac = BC, ≌ - ADC ≌ - CEB (AAS)