多項式的因式分解~急~

1)
2y(3-4a)-(4a-3)
=2y(3-4a)+(3-4a)
=(3-4a)(2y+1)

2)
(2+3c)^2 - (2c+4)^2
=[(2+3c)-(2c+4)][(2+3c)+(2c+4)]
=(2+3c-2c-4)(2+3c+2c+4)
=(c-2)(5c+6)

3)
81-u^4
=(9-u^2)(9+u^2)
=(3-u)(3+u)(9+u^2)

4)
(a-b)^2 + 6(a-b)+9
=[(a-b)+3]^2
=(a-b+3)^3

5)
4-20(c-3d)+25(c-3d)^2
=25(c-3d)^2-20(c-3d)+4
=[5(c-3d)+2]^2
=(5c-15d+2)^2

唔知點解都係錯T.T
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2011-12-31 15:32:12 補充：
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Useful formulas

a^2 - b^2 = (a - b)(a + b)

a^2 + 2ab + b^2 = (a + b)^2

a^2 - 2ab + b^2 = (a - b)^2

1 2y(3 - 4a) - (4a - 3)

= -2y(4a - 3) - (4a - 3)

= -(4a - 3)(2y + 1)

2 (2 + 3c)^2 - (2c + 4)^2

= (2 + 3c + 2c + 4)(2 + 3c - 2c - 4)

= (5c + 6)(c - 2)

3 81 - u^4

= 9^2 - (u^2)^2

= (9 + u^2)(9 - u^2)

= (9 + u^2)(3 + u)(3 - u)

4 (a - b)^2 + 6(a - b) + 9

=(a - b + 3)^2

5 4 - 20(c - 3d) + 25(c - 3d)^2

= [2 - 5(c - 3d)]^2

= (2 - 5c + 15d)^2