∵AE平分∠BAD,
∴∠BAE=∠DAE,
又∵AD∥CB,
∴∠AEB=∠DAE,
∴∠BAE=∠AEB,
则BE=AB=5;
同理可得,CF=CD=5.
∴EF=BE+CF-BC=BE+CF-AD=5+5-8=2.
故填空答案:2.