Because B falls on E with fg as the broken line, fg is perpendicular to be, and the intersection of fg and be is O, so ob=oe, so triangle bof and triangle bae are similar triangles, so BO/of = AB/AE; Because ae=5, ab= 12, be= 13 and bo=6.5, of=65/24, BF =169/24; A straight line passing through G point and parallel to bc intersects H point. Because the triangles ghf and BOF are both right-angled triangles and similar triangles, FG/Hg = BF/BO; So fg=bf*hg/bo=65/6, as shown in the figure.