题中条件,∠ABC=90°,改为:∠ACB=90°
∠ACD=3∠BCD=(3/4)90°,∠BCD=(1/4)90°
∠ECD=∠ACD-∠ACE
=∠ACD-∠CAE
=∠ACD-∠BCD
=(3/4)90-(1/4)90°
=45°