1/1*2*3+1/2*3*4+1/98*99*100
=0.5(1/1*2-1/2*3)+0.5(1/2*3-1/3*4)+0.5(1/3*4-1/4*5)+…+0.5(1/98*99-1/99*100)
=0.5(1/1*2-1/99*100)
=4949/19800
1/1*2*3+1/2*3*4+1/98*99*100
=0.5(1/1*2-1/2*3)+0.5(1/2*3-1/3*4)+0.5(1/3*4-1/4*5)+…+0.5(1/98*99-1/99*100)
=0.5(1/1*2-1/99*100)
=4949/19800