Đáp án:
$a. 13x^{2} - 13y^{2} = 13( x^{2} - y^{2} )$
⇔ $13x^{2} - 13y^{2} = 13( x - y )( x + y )$
$b. 14xy - 35x^{3}y + 28x^{2}y^{2} = 7xy( 2 - 5x^{2} + 4xy )$
$c. 3x^{3} + 18x^{2}y + 27xy^{2} = 3x( x^{2} + 6xy + 9y^{2} )$
⇔ $3x^{3} + 18x^{2}y + 27xy^{2} = 3x( x + 3y )^{2}$
$d. x^{3} - 2x^{2} + x = x( x^{2} - 2x + 1 )$
⇔ $x^{3} - 2x^{2} + x = x( x - 1 )^{2}$
$e. 2x( x + 1 ) + 6( x + 1 ) = 2( x + 1 )( x + 3 )$
$i. x^{2} + xy - 8x - 8y = ( x^{2} + xy ) - ( 8x + 8y )$
⇔ $x^{2}y + xy - 8x - 8y = x( x + y ) - 8( x + y )$
⇔ $x^{2}y + xy - 8x - 8y = ( x + y )( x - 8 )$
