`x + 2/( 1 . 5 ) + 2/( 5 . 9 ) + .... + 2/( 101 . 105 ) = - 1999 1/210`
`⇔ 2x + 4/( 1 . 5 ) + 4/( 5 . 9 ) + .... + 4/( 101 . 105 ) = - 3998 1/210 . 2`
`⇔ 2x + 1/1 - 1/5 + 1/5 - 1/9 + .... + 1/101 - 1/105 = - 3998 1/105`
`⇔ 2x + 1 - 1/105 = - 3998 1/105`
`⇔ 2x + 104/105 = - 3998 1/105`
`⇔ 2x = - 3998 1/105 - 104/105`
`⇔ 2x = - 3999`
`⇔ x = - 3999 : 2`
`⇔ x = ( - 3999 )/2`
Vậy `, x = ( - 3999 )/2 .`