3x( y + 2 ) - 3( 1 - 2x ) ( như này đúng k -..- )
= 3( xy + 2x ) - 3( 1 - 2x )
= 3[ xy + 2x - ( 1 - 2x ) ]
= 3( xy + 2x - 1 + 2x )
= 3( xy + 4x - 1 )
x2 - y2 - 2x + 2y
= ( x2 - y2 ) - 2( x - y )
= ( x - y )( x + y ) - 2( x - y )
= ( x - y )( x + y - 2 )
2x + 2y - x2 - xy
= 2( x + y ) - x( x + y )
= ( x + y )( 2 - x )