`x . ( 4/( 1 . 2 ) + 4/( 2 . 3 ) + 4/( 3 . 4 ) + ... + 4/( 99 . 100 ) ) = 33/25`
`⇔ x . 4 . ( 1/( 1 . 2 ) + 1/( 2 . 3 ) + 1/( 3 . 4 ) + ... + 1/( 99 . 100 ) ) = 33/25`
`⇔ x . 4 . ( 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/99 - 1/100 ) = 33/25`
`⇔ x . 4 . ( 1 - 1/100 ) = 33/25`
`⇔ x . 4 . 99/100 = 33/25`
`⇔ x . 99/25 = 33/25`
`⇔ x = 33/25 : 99/25`
`⇔ x = 1/3`
Vậy , `x = 1/3 .`
