A = `1/{10 x 11}` + `1/{11 x 12}` + `1/{12 x 13}` + `1/{3 x 14}` + .... + `1/{99 x 100}`
A = 1 - ( `1/10` - `1/11` + `1/11` - `1/12` + `1/12` - `1/13` + `1/13` - `1/14` + ... + `1/99` - `1/100` )
A = `1/10` - `1/100`
A = `9/100` = `0,09`
Mik xin hay nhất ạ !