$\\$
`1,`
`A=1/(1.2)+1/(2.3)+... + 1/(98 . 99)+1/(99.100)`
`->A=1-1/2+1/2-1/3+...+1/98-1/99+1/99-1/100`
`->A=1-1/100`
`->A=100/100-1/100`
`->A=99/100`
Vậy `A=99/100`
$\\$
`2,`
Đặt `B = 9/(19.29)+9/(29.39)+...+9/(1999 . 2009)`
`->B = 9/10 (1/19 - 1/29 + 1/29-1/39 + ... + 1/1999 - 1/2009)`
`->B = 9/10 . (1/19 - 1/2009)`
`->B = 9/10 . 1/38171`
`->B= 1791/38171`
`C=1/19 + 9/(19.29)+9/(29.39)+...+9/(1999 . 2009) )`
`->C=1/19 + (9/(19.29)+9/(29.39)+...+9/(1999 . 2009) )`
`->C=1/19 + 1791/8171`
`->C=200/2009`
Vậy `C=200/2009`
