Bài `2` `c :`
`8/19 . 5/11 + 7/19 . 8/11 - 8/19 . 1/11`
`= 8/19 . ( 5/11 - 1/11 ) + 7/19 . ( 2 . 4 )/11`
`= 8/19 . 4/11 + 7/19 . 2 . 4/11`
`= 8/19 . 4/11 + 14/19 . 4/11`
`= 4/11 . ( 8/19 + 14/19 )`
`= 4/11 . 22/19`
`= 8/19`
Bài `3` `b :`
`1/( 1 . 3 ) + 1/( 3 . 5 ) + 1/( 5 . 7 ) + .... + 1/( x . ( x + 2 ) ) = 1005/2011`
`⇔ 1/1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + .... + 1/x - 1/( x + 2 ) = 1005/2011`
`⇔ 1 - 1/( x + 2 ) = 1005/2011`
`⇔ 1 : ( x + 2 ) = 1 - 1005/2011`
`⇔ 1 : ( x + 2 ) = 1006/2011`
`⇔ x + 2 = 1 : 1006/2011 `
`⇔ x + 2 = 2011/1006`
`⇔ x = 2011/1006 - 2`
`⇔ x = ( - 1 )/1006`
Vậy , `x = ( - 1 )/1006`
