Because the unit of AD⊥MN is D, ∠ ACD+∠ DAC = 90.

So ∠BEC=∠DAC

So ∠ACD=∠CBE

Because AC=BC

So △ADC is equal to △CEB.

So AD=CE DC=EB

Because DE=DC+CE

So DE=AD+BE

(2)△ADC is equal to△△ CEB, but DE=AD+BE is not.

Because ACB = 90 DCA+ECB = 90.

Because AD⊥MN is in D, ∠ DCA+∠ CAD = 90.

So ∠ECB=∠CAD.

So ∠ACD=∠CBE

Because AC=CB

So △ADC is equal to △CEB.