=> ( 2x + 4x + ... + 400x ) - ( 1 + 2 + ... + 200 ) = 5.( 1 + 2 + ... + 200 )
=> x.{ [ 400 + 2 ] . 200 ] : 2 } - { [ 200 . ( 200 + 1 ) ] : 2 } = 5.{ [ 200 . ( 200 + 1 ) ] : 2 }
=> x.40200 - 20100 = 100500
=> 40200x = 120600 => x = 120600 : 40200 = 3
Vậy x = 3