1乘以2乘以3分之1+2乘以3乘以4分之1+……+98乘以99乘以100分之1
=[1/(1x2)-1/(2x3)+1/(2x3)-1/(3x4)+……+1/(98x99)-1/(99x100)]÷2
=[1/(1x2)-1/(99x100)]÷2
=[1/2-1/9900]÷2
=4949/9900÷2
=4949/19800