`A= 1/(1.2.3) + 1/(2.3.4) + ...+ 1/(98.99.100)`
`A=1/2(2/(1.2.3) + 2/(2.3.4) +...+ 2/(98.99.100))`
`A=1/2(1/1.2-1/2.3+1/2.3-1/3.4+...+1/98.99-1/99.100)`
`A=1/2(1/1.2-1/99.100)`
`A=1/2(1/2-1/9900)`
`A=1/2(4950/9900-1/9900)`
`A=1/2. 4949/9900`
`A=4949/19800`
Xin hay nhất ạ ^_^