F.2 Maths

7. x^5 - -81x
= x(x^4 - 9^2)
= x (x^2 +9)(x^2 - 3^2)
= x (x^2 +9)(x+3)(x-3)

8. x^2 - 2x + 1 - y^2
= (x-1)^2 - y^2
= (x-1+y)(x-1-y)

9. x^2 + 8x + 16 - 25y^
= (x+4)^2 - (5y)^2
= (x+4+5y)(x+4-5y)

10. 63x^2 - 7x^4
= 7x^2 [3^2 - x^2]
= 7x^2 (3+x)(3-x)

2007-10-20 17:07:57 補充：
7. x^5 - -81x 應該係 x^5 - 81x 嗎?9. x^2 8x 16 - 25y^ 應該係 x^2 8x 16 - 25y^2 嗎?以上問題係用 (a-b)^2 = (a b)(a-b)