2x^2+9x-5
這條是否用甚麼(自已叫)「cross-method」,即
1 -1
2 -5,
1 -5
2 -1
.........
????
請各位幫幫忙忙~
此外,我想問幾時要加個(-1)?
即:(2-1)+(1-2)
=(2x-1)y+x(-1)(2x-1)
------------------------------
thx:)
(10點)!! F.2 maths
2013-12-01 9:23 pm
回答 (2)
2013-12-01 11:53 pm
✔ 最佳答案
十字相乘法英文(http://tw.knowledge.yahoo.com/question/question?qid=1306031713622)Cross method
或是
Cross-product method
十字相乘法
http://baike.baidu.com/view/198055.htm
Eg.
2x²+9x-5
=(2x-1)(x+5)
_________________________
2x╲╱-1
_x╱╲+5
────
-x+10x=9x
↑
2x(x)=2x²(左方)
-x-10x=9x(下方)
-1(5)=-5(右方)
2x²+9x-5
2013-12-01 16:00:14 補充：
當2x²+9x-5
常数就只需嘗試:
2个数之积为-ve
如Eg.
即
-5,1
OR
-1,5
而
-5,-1
OR
1,5
之积为+ve
LAST YOU WILL KNOW
-5,1✘
-1,5✔
-5,-1✘
1,5✘
參考: me
2013-12-01 11:58 pm
2x^2+9x-5
答案一定係(2x+a)(x+b)先會出到2x^2, where a*b = -5
可能性：
(1) a = 5 ; b = -1
(2) a = -1 ; b = 5
只要出到9x 就得
For (1) ---> 5*x + (-1)(2x) = 3x ----> WRONG
For (2) ---> (-1)x + 5*(2x) = 9x -----> CORRECT
THEREFORE,
2x^2+9x-5
=(2x-1)(x+5)
答案一定係(2x+a)(x+b)先會出到2x^2, where a*b = -5
可能性：
(1) a = 5 ; b = -1
(2) a = -1 ; b = 5
只要出到9x 就得
For (1) ---> 5*x + (-1)(2x) = 3x ----> WRONG
For (2) ---> (-1)x + 5*(2x) = 9x -----> CORRECT
THEREFORE,
2x^2+9x-5
=(2x-1)(x+5)
收錄日期: 2021-04-13 19:50:12
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20131201000051KK00071