$B=\dfrac{1}{1.2.3}+\dfrac{1}{2.3.4}+..+\dfrac{1}{98.99.100}$
`⇒ 2B=2/(1.2.3)+2/(2.3.4)+....+2/(98.99.100)`
`⇒ 2B=1/(1.2)-1/(2.3)+1/(2.3)-1/(3.4)+....+1/(98.99)-1/(99.100)`
`⇒ 2B=1/(1.2)-1/(99.100)`
`⇒ 2B=1/2-1/9900`
`⇒ 2B=4949/9900`
`⇒ B=4949/19800`
Xin hay nhất !