mathsf2,,

圖片參考：http://i.imagehost.org/0335/ScreenHunter_04_Apr_12_15_51.gif


http://i.imagehost.org/0335/ScreenHunter_04_Apr_12_15_51.gif

2010-04-12 21:35:24 補充：
另外2題:
http://i.imagehost.org/0428/ScreenHunter_05_Apr_12_21_34.gif

1. 係咪應該係identity(即中間等號應該是三劃)?

(3x-1) ( x-a ) = 3x^2 +bx -2
3x^2 - 3xa - x + a = 3x^2 +bx -2 ──expand (3x-1) ( x-a )
3x^2 - (3a+1)x + a = 3x^2 +bx -2 ──group like terms
By comparing coefficients,
a = -2,
b = -(3a+1) ── Substitute a=-2
= -(3*-2+1)
= 5

2. a=(1+b)/(1-b)
(1-b)a=1+b ──multiply both sides by (1-b)
a-ab=1+b ──expand (1-b)a
ab+b=a-1 ──group like terms
b(a+1)=a-1
b=(a-1)/(a+1) ──divide both sides by (a+1)

3. 注意正負號, 應該是 (x-y)(x+y-1)
x^2 - y^2 -x+y
=(x+y)(x-y) -x+y ──By identity a^2-b^2 = (a+b)(a-b)
=(x+y)(x-y) -(x-y) ──*Take out -1 from -x+y to form -(x-y)
=(x-y)(x+y-1) ──factorization

2010-04-12 15:57:30 補充：
右手邊的只是explanation,
不用答落去的.