△ABC~△AED,DE=1(正方形),BC=3,DE: CB=1:3DE:CB=AE:AB1:3=AE:4, 3AE=4, AE=4/3△ABN~△AFG,AF:AB=FG:BN=(7/3): 4=1: BNBN=4(3/7)=12/7#