a) ( 2x + 1 )2 + 10( 2x + 1 ) + 25
= ( 2x + 1 )2 + 2.( 2x + 1 ).5 + 52
= [ ( 2x + 1 ) + 5 ]2
= ( 2x + 1 + 5 )2
= ( 2x + 6 )2
b) x2 + 2x( y - 2 ) + y2 - 4y + 4
= x2 + 2x( y - 2 ) + ( y2 - 4y + 4 )
= x2 + 2x( y - 2 ) + ( y - 2 )2
= [ x + ( y - 2 ) ]2
= ( x + y - 2 )2
c) x2 + 12x + 40 + y2 + 4y
= ( x2 + 12x + 36 ) + ( y2 + 4y + 4 )
= ( x + 6 )2 + ( y + 2 )2 ( cấy ni không viết được ;-; )
d) x2 - 8x - 20 - y2 - 12y
= ( x2 - 8x + 16 ) - ( y2 + 12y + 36 )
= ( x - 4 )2 - ( y + 6 )2
= [ ( x - 4 ) - ( y + 6 ) ][ ( x - 4 ) + ( y + 6 ) ]
= ( x - 4 - y - 6 )( x - 4 + y + 6 )
= ( x - y - 10 )( x + y + 2 )
e) x2 + y2 + 4x + 4y + 2( x + 2 )( y + 2 ) + 8
= ( x2 + 4x + 4 ) + 2( x + 2 )( y + 2 ) + ( y2 + 4y + 4 )
= ( x + 2 )2 + 2( x + 2 )( y + 2 ) + ( y + 2 )2
= [ ( x + 2 ) + ( y + 2 ) ]2
= ( x + 2 + y + 2 )2
= ( x + y + 4 )2