设BF=x,
由折叠的性质可得:BF=BF′=x,
∵FB′ ∥ AB,
∴
B′F
AB =
FC
BC ,
∵AB=AC=3,BC=4,
∴FC=BC-BF=4-x,
∴
x
3 =
4-x
4 ,
解得:x=
12
7 .
故答案为:
12
7 .
设BF=x,
由折叠的性质可得:BF=BF′=x,
∵FB′ ∥ AB,
∴
B′F
AB =
FC
BC ,
∵AB=AC=3,BC=4,
∴FC=BC-BF=4-x,
∴
x
3 =
4-x
4 ,
解得:x=
12
7 .
故答案为:
12
7 .