2/1*2+2/2*3+2/3*4+2/4*5+....2/99*100=2*(1/1*2+1/2*3+1/3*4+.......+1/99*100)=2*(1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+.......1/98-1/99+1/99-1/100)=2*(1-1/100)=2*99/100=99/50=50分之99
