Given, In △ABC, ∠A=90 and AD⊥BC
In △ABC,
∠BAC+∠ABC+∠ACB=180
90+∠ABC+∠ACB=180
∠ABC+∠ACB=90 (I)
In △CAD,
∠CAD+∠ACD+∠ADC=180
∠CAD+∠ACD+90=180
∠CAD+∠ACD=90..(II)
Equating (I) and (II),
∠ABC+∠ACB=∠CAD+∠ACD
∠ABC=∠CAD...(III)
Similarly, ∠ACB=∠BAD...(IV)
Now, In △s, ABD and CAD
∠ABC=∠CAD...(From III)
∠BAD=∠ACB ..(From IV)
∠ADB=∠ADC (Each 90∘)
Thus, △ADB∼△CAD (AAA rule)