唔識做功課10

Let u = x² - 4x

(x² - 4x) - 17(x² - 4x) - 84
= u² - 17u - 84
= (u + 4)(u - 21)
= (x² - 4x + 4) (x² - 4x - 21)
= (x - 2)²(x + 3)(x- 7)

2013-04-12 23:07:07 補充：
Or:
(x² - 4x) - 17(x² - 4x) - 84
= [(x² - 4x) + 4] [(x² - 4x) - 21]
= (x² - 4x + 4) (x² - 4x - 21)
= (x - 2)²(x + 3)(x - 7) ...... (答案)

2013-04-13 00:38:08 補充：
Or:
(x² - 4x) - 17(x² - 4x) - 84
= [(x² - 4x) + 4] [(x² - 4x) - 21]
= (x² - 4x + 4) (x² - 4x - 21)
= (x - 2)²(x + 3)(x - 7) ...... (答案)