设BE=x,则ED=3x,
∵∠ABE+∠BAE=90°,∠ABD+∠ADB=90°,
∴∠BAE=∠ADE,
∵∠AEB=∠AED,
∴△ABE∽△DBA,
∴
=
,∴AB 2=BE×BD,
即36=x(x+3x),
解得x=3,BD=3×(1+3)=12,故AC=BD=12.
设BE=x,则ED=3x,
∵∠ABE+∠BAE=90°,∠ABD+∠ADB=90°,
∴∠BAE=∠ADE,
∵∠AEB=∠AED,
∴△ABE∽△DBA,
∴
=
,∴AB 2=BE×BD,
即36=x(x+3x),
解得x=3,BD=3×(1+3)=12,故AC=BD=12.