( x + 1 ) + ( x + 2 ) + ( x + 3 ) + . . . + ( x + 2009 ) = 2009 . 2010
x . 2009 + ( 1 + 2 + 3 + . . . + 2009 ) = 2009 . 2010
x . 2009 + [ ( 2009 + 1 ) . 2009 : 2 ] = 2009 . 2010
x . 2009 + 1005 . 2009 = 2009 . 2010
2009 ( x + 1005 ) = 2010
x + 1005 = 2010
x = 1005
Vậy x = 1005