1/2X3+1/3X4+1/4X5+.....+1/98X99+1/99X100=1/2-1/3+1/3-1/4+1/4-1/5+。。。+1/98-1/99+1/99-1/100=1/2-1/100=49/100
13x4/7x14=5/13x8=40/13