s=1/6
S△DEF=S正方形ABCD-S△DCE-S△ABE-S△DAF
因为△BEF∽△DAF
且BE:AD=1:2,所以△DAF的高为2/3
所以 S△DEF=1X1-1/2x1x0.5-1/2x1x0.5-1/2x1x2/3=1/6