中二math 一問

你好～～～

以下都是本人的一些意見，如有錯誤請多多指教～～

1.(3x-2y)(-2y-3x)

= (3x - 2y)(-1)(2y + 3x)

= -[(3x - 2y)(3x + 2y)]

= -[(3x)^2 - (2y)^2]

= -(9x^2 - 4y^2)

2.-2(8s+3t)(3t-8s)

= -2(3t + 8s)(3t - 8s)

= -2[(3t)^2 - (8s)^2]

= -2(9t^2 - 64s^2)

= -18t^2 + 128s^2

= 128s^2 - 18t^2

3.3(-9h-2k)(2k-9h)

= 3(-1)(9h + 2k)(2k - 9h)

= -3(2k + 9h)(2k - 9h)

= -3[(2k)^2 - (9h)^2]

= -3(4k^2 - 81h^2)

= -12k^2 + 243h^2

= 243h^2 - 12k^2

4.(3r/2 + 7/2)(3r/4 - 7/4)

= (3r/2 + 7/2)[(1/2)(3r/2 - 7/2)]

= (1/2)(3r/2 + 7/2)(3r/2 - 7/2)

= (1/2)[(3r/2)^2 - (7/2)^2]

= (1/2)(9r^2 / 4 - 49 / 4)

= (1/2)[(9r^2 - 49)/4]

= (9r^2 - 49) / 8

5. ( 8/7 - 4m/7)(8/5 + 4m/5)

= (8/7)(8/5) + (8/7)(4m/5) - (4m/7)(8/5) - (4m/7)(4m/5)

= 64 / 35 + 32m / 35 - 32m / 35 - 16m^2 / 35

= -(16m^2 / 35 - 64 / 35)

6. 8(a/2 + b/4)(a/2 - b/4)

= 8[(a/2)^2 - (b/4)^2]

= 8(a^2 / 4 - b^2 / 16)

= 8[(4a^2 - b^2) / 16]

= (4a^2 - b^2) / 2

7.(7n+3m)(7n-3m)

= (7n)^2 - (3m)^2

= 49n^2 - 9m^2

8. 21(n-3m/7)(m+7n/3)

= 21(n - 3m/7)(m + 7n/3)

= 21[(7n - 3m) / 7][(3m + 7n) / 3]

= (7n - 3m)(3m + 7n)

= (7n - 3m)(7n + 3m)

= (7n)^2 - (3m)^2

= 49n^2 - 9m^2

9.(8m+3n)(3n-8m)

= (3n + 8m)(3n - 8m)

= (3n)^2 - (8m)^2

= 9n^2 - 64m^2

10.(m+ 3/8n)(n- 8/3n)

這題好像怪怪的呢～～

要記得一條公式︰

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

希望可以幫到你～～～～～

1. (3x-2y)(-2y-3x)
=(3x-2y)(-2y)+(3x-2y)(-3x)
=(-6xy2-4y2)+(-9x2-6xy)
= -6xy2-4y2-9x2-6xy
= -6xy2-6xy-4y2-9x2
= -12x2y3-4y2-9x2
咁？？？？？？
唔明

2010-09-27 19:35:52 補充：
唔明唔明唔明唔明唔明唔明唔明唔明唔明

...5G Arrr...