设AD=DE=EB=a,AE=DB=2a,AB=3a由正弦定理,有CD/sinA=AD/sin∠ACDsinA/sinA=a/sin∠ACD得sin∠ACD=aCE/sinB=EB/sin∠ECBcosA/sin(π/2-A)=a/sin∠ECBcosA/cosA=a/sin∠ECB得sin∠ECB=a∵∠ACD,∠ECB都是锐角∴∠ACD=∠...