9) x⁴ - 10x² + 9
= x⁴ - x² - 9x² + 9
= x² (x² - 1) - 9 (x² - 1)
= (x² - 9) (x² - 1)
= (x - 3)(x + 3)(x - 1)(x + 1)
10) x² - x - 20
= x² + 4x - 5x - 20
= x (x - 4) - 5 (x - 4)
= (x - 5)(x - 4)
11) x² - xy + x - y
= (x² - xy) + (x - y)
= x (x - y) + (x - y)
= (x + 1)(x - y)
12) 3x² - 3xy - 5x + 5y
= (3x² - 3xy) - (5x - 5y)
= 3x (x - y) - 5 (x - y)
= (3x - 5)(x - y)
13) 5a² - 5ax - 7a + 7x
= (5a² - 5ax) - (7a - 7x)
= 5a (a - x) - 7(a -x)
= (5a - 7)(a - x)
