Bài 4:
1) x² + 4x - y² + 4
= (x² + 4x + 4) - y²
= (x + 2)² - y²
= (x + 2 + y) . (x + 2 - y)
2) x² + xy - 10x - 10y
= (x² + xy) - (10x + 10y)
= x . (x + y) - 10 . (x + y)
= (x + y) . (x - 10)
3) 7x² - 7xy - 10x + 10y
= (7x² - 7xy) - (10x - 10y)
= 7x . (x - y) - 10 . (x - y)
= (x - y) . (7x - 10)
4) 5a² - 5ax - 8a + 8x
= (5a² - 5ax) - (8a - 8x)
= 5a . (a - x) - 8 . (a - x)
= (a - x) . ( 5a - 8)
5) 10ax - 5ay + 2x - y
= (10ax - 5ay) + (2x - y)
= 5a . (2x - y) + (2x - y)
= (2x - y) . (5a + 1)
6) 2x² - 6xy + 5x - 15y
= (2x² - 6xy) + (5x - 15y)
= 2x . (x - 3y) + 5 . (x - 3y)
= (x - 3y) . (2x + 5)
7) x . (a + b) - a - b
= x . (a + b) - (a + b)
= (a + b) . (x - 1)
8) a . (x - y) - x + y
= a . (x - y) - (x - y)
= (x - y) . (a - 1)
9) x² + 2xy + y² - 25
= (x² + 2xy + y²) - 25
= (x + y)² - 5²
= (x + y + 5) . (x + y - 5)
10) x² - 4xy + 4y² - 100
= (x² - 4xy + 4y²) - 100
= (x - 2y)² - 10²
= (x - 2y + 10) . (x - 2y - 10)
