设ad=4x则可求bd=dc=3x ab=ac=5x ae=2x ec=3x
过e做dc的垂线f
ef/ad=ce/ca得 ef=x*12/5 cf/cd=ce/ca 得x*9/5 df=3x-x*9/5 =x*6/5
tan∠ADE=tandef=df/ef=x*6/5/x*12/5 =1/2