In△DAB,Let lineEFintersect sideDBatG, such thatE−G−Fis a line.Eis the midpoint of sideADandEG∥AB.By converse of midpoint theorem,Gis the midpoint of sideBD (1)AB∥DCgivenAB∥EFgiven∴AB∥DC∥EF(2)In△BDC,Gis the midpoint of sideBD...from (1)andDC∥GFfrom (2)By converse of midpoint theorem,Fis the midpoint of sideBC