`( x + 1 ) + ( x + 4 ) +...+ ( x + 1996 ) = 998001`
`⇒ ( x + x + x + ... + x ) + ( 1 + 4 +... + 1996 ) = 998001`
`⇒ 666x + ( 1996 + 1 ) xx 666 : 2 = 998001`
`⇒ 666x + 1997 xx 333 = 998001`
`⇒` 666x + 665001 = 998001
`⇒ 666x = 998001 - 665001`
`⇒ 666x = 333000
`⇒ x = 333000 : 666`
`⇒ x = 500`
Vậy `x = 500`