∵AD∥EF∥BC,∴AE/BD=DF/CF,(平行线分线段成比例)
∵AD∥BC,∴ΔADE∽ΔBGF,ΔADF∽ΔHCF,
∴AD/BG=AE/BE,AD/CH=DF/CF,
∴AD/BG=AD/CH
∴BG=CH.