`( 3x )/( 2 . 5 ) + ( 3x )/( 5 . 8 ) + ( 3x )/( 8 . 11 ) + ( 3x )/( 11 . 14 ) = 1/21`
`⇔ x . ( 3/( 2 . 5 ) + 3/( 5 . 8 ) + 3/( 8 . 11 ) + 3/( 11 . 14 ) ) = 1/21`
`⇔ x . ( 1/2 - 1/5 + 1/5 - 1/8 + 1/8 - 1/11 + 1/11 - 1/14 ) = 1/21`
`⇔ x . ( 1/2 - 1/14 ) = 1/21`
`⇔ x . 3/7 = 1/21`
`⇔ x = 1/21 : 3/7`
`⇔ x = 1/9`
Vậy `, x = 1/9 .`