解?$ ∶ (1) ①$?∵?$∠ACD+∠BCE=90°$?，
?$∠DAC+∠ACD=90°$?
∴?$∠DAC=∠BCE$?
又∵?$AC=BC$?，?$∠ADC=∠BEC=90°$?
∴?$\triangle A D C≌\(chéng)triangle C E B $?
②∵?$\triangle A D C≌\(chéng)triangle C E B$?
∴?$C D=B E$?，?$A D=C E$?
∴?$D E=C E+C D=A D+B E $?
?$(2) \triangle ADC≌\(chéng)triangle CEB $?成立，?$DE=AD+BE$?不成立?$ $?
此時(shí)應(yīng)有?$D E=A D-B E$?
∵?$∠ACD+∠BCE=90°$?，?$∠DAC+∠ACD=90°$?
∴?$∠DAC=∠BCE$?
?$ $?又?$AC=BC$?，?$∠ADC=∠BEC=90°$?
∴?$\triangle ADC ≌\(chéng)triangle CEB$?
∴?$C D=B E$?，?$A D=C E$?
∴?$D E=A D-B E$?
