$#Dứa$
a, x² + 4xy - 21y²
= x² +7xy - 3xy - 21y²
= x (x + 7y) - 3y (x + 7y)
= (x - 3y) (x + 7y)
b, 5x² + 6xy + y²
= 5x² + 5xy + xy + y²
= 5x (x + y) + y (x + y)
= (5x + y) (x + y)
c, x² + 2xy - 15y²
= x² + 5xy - 3xy + 15y²
= x (x + 5y) - 3y (x + 5y )
= (x - 3y) (x + 5y)
d, (x - y)² + 4 (x - y) - 12
= (x - y)² - 2 (x - y) + 6 (x - y) - 12
= (x - y) (x - y - 2) + 6 (x - y - 2)
= (x - y - 2) (x - y + 6)
e, x² - 7xy + 10y²
= x² - 5xy - 2xy - 10y²
= x (x - 5y) - 2y (x - 5y)
= (x - 2y) (x - 5y)
f, x²yz + 5xyz - 14yz
= yz (x² + 5x - 14)
= yz (x² - 2x + 7x - 14)
= yz [x (x - 2) + 7 (x - 2)]
= yz (x - 2) (x + 7)
