Bài 16 :
a) x³ - 4x² - 2x + 8
= x² ( x - 4 ) - 2 ( x - 4 )
= ( x² -2 ) ( x - 4 )
b) 4x - 4 - x² + 9y²
= 9y² - ( x² - 4x + 4 )
= 9y² - ( x² - 2 . x . 2 + 2² )
= ( 3y )² - ( x - 2 ) ²
= ( 3y + x - 2 ) ( 3y - x + 2 )
c) 49y² - 9x² + 6x - 1
= 49y² - ( 9x² - 6x + 1 )
= ( 7y )² - [ ( 3x )² - 2 . 3x . 1 + 1² ]
= ( 7y )² - ( 3x - 1 )²
= ( 7y + 3x - 1 ) ( 7y - 3x +1 )
d) x² - 4x - 4y² + 8y
= x² + 2xy - 4x - 2xy -4y² + 8y
= x ( x + 2y - 4 ) - 2y ( x + 2y - 4 )
= ( x - 2y ) ( x + 2y - 4 )
Bài 17 :
a) x² - 25 - 10y - y²
= x² - ( y² + 10y + 25 )
= x² - ( y² + 2 . y . 5 + 5² )
= x² - ( y + 5 )²
= ( x + y + 5 ) ( x - y -5 )
b) x³ - 6x²- 9x + 54
= x² (x - 6 ) - 9 ( x - 6 )
= ( x² - 9 ) ( x - 6 )
= ( x + 3 ) ( x - 3 ) ( x - 6 )
c) x³ + 3x² - 4x - 12
= x² ( x + 3 ) - 4 (x + 3 )
= ( x² - 4 ) ( x + 3 )
= ( x+ 2 ) ( x -2 ) ( x + 3 )
d) 5 ( x - y ) - y ( y - x )
= 5 ( x - y ) + y ( x - y )
= ( 5 + y ) ( x - y )
CHÚC HỌC TỐT !!!