BE长为6.
设BE长为x,AB为h
(S为面积),
∵AB=CD=h
∴S△ABE=xh/2
S梯形AECD=(AD+EC)×CD÷2=(36-x)h/2
又∵S△ABE=1/5×S梯形AECD
(36-x)h=5xh
36h=6xh
x=6