(1), because angle ACB=90 degrees, angle DCA+ angle ECB=90 degrees, angle CDA=90 degrees, angle DCA+ angle CAD=90 degrees, so angle CAD= angle ECB, and because angle ADC= angle BE=DC=90 degrees, AC=AB, so triangle ADC and triangle CEB.

(2) Because angle BCE+ angle DCA=90 degrees and angle DCA+ angle CAD=90 degrees, angle BCE= angle CAD, and because angle ADC= angle BEC=90 degrees and AC=BC, triangle ACD and triangle CBE are congruent, so AD=CE, CD=BE, so DE = CE-CD = AD.

(3) Prove that triangle CAD and triangle BCE are congruent as in (2),

So CD=BE, CE=AD, so DE=CD-CE=BE-AD.