B = ( A + 3B ) ( C - D ) - ( 3A - D ) ( B+ C) - 2C ( B - A ) + 2B ( A+ D )
   = AC - AD + 3BC - 3BD - 3 AB - 3AC + DB + CD - 2CB + 2AC + 2AB + 2BD
   = BC - AD - AB + CD = ( BC + CD ) - ( AB + AD )
   = C ( B + D ) - A ( B + D ) = ( C - A ) ( B + D )
 CHÚC BN HC TỐT Ạ XIN CTLHN