1/1*2*3+1/2*3*4+1/3*4*5+……1/98*99*100
=1/2[1/1*2-1/2*3]+1/2[1/2*3-1/3*4]+.+1/2[1/98*99-1/99*100]
=1/2[1/1*2-1/2*3+1/2*3-1/3*4+.+1/98*99-1/99*100]
=1/2[1/1*2-1/99*100]
=1/2*4949/9900
=4949/19800