定義
在一個含有若干個元的多項式中,如果任意交換兩個元的位置,多項式不變,這樣的多項式叫做對稱多項式。
輪換式與對稱式
如果一個多項式中的變數字母按照任何次序輪換後,原多項式不變,那么稱該多項式是輪換多項式。 對稱輪換式就是兩項互換,可以看做輪換式的特殊情況。
下面通過例子來說明輪換式和對稱式的區別和聯繫:
![輪換式](/img/1/a80/wZwpmLzgTMxkDO1ITOwMzM1UTM1QDN5MjM5ADMwAjMwUzLykzL4IzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
1、對於這幾個代數式:
交換這些式子中的任意兩個字母,式子不變。我們把這樣的式子叫做 對稱式。
![輪換式](/img/2/a60/wZwpmL3UDNwMDN0gjNwMzM1UTM1QDN5MjM5ADMwAjMwUzL4YzL0czLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
2、再看這幾個式子:
將這些式子中的 x 換成 y, 將 y 換成 z, 將 z 換成 x,即將字母做一個輪換, 式子保持不變。我們將這樣的式子叫做 輪換式。
由此可見: 對稱式一定是輪換式,但輪換式未必是對稱式。另外,兩個輪換式(對稱式)的和、差、積、商仍然是輪換式(對稱式)。
輪換式的因式分解
![輪換式](/img/f/58b/wZwpmL0UjM2MTO3YzNxMzM1UTM1QDN5MjM5ADMwAjMwUzL2czL4AzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
例1:分解因式
![輪換式](/img/a/d3c/wZwpmLzUTNwATM2kTOxMzM1UTM1QDN5MjM5ADMwAjMwUzL5kzLwYzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![輪換式](/img/a/d3c/wZwpmLzUTNwATM2kTOxMzM1UTM1QDN5MjM5ADMwAjMwUzL5kzLwYzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![輪換式](/img/a/39e/wZwpmLzYjNwczMzETOxMzM1UTM1QDN5MjM5ADMwAjMwUzLxkzL2UzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![輪換式](/img/a/d3c/wZwpmLzUTNwATM2kTOxMzM1UTM1QDN5MjM5ADMwAjMwUzL5kzLwYzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
分析: 這是一個二元對稱式,二元對稱式的基本對稱式是 .任何二元對稱多項式都可用 表示,如 ,二元對稱多項式的分解方法之一是:先將原式用 表示,再行分解。
![輪換式](/img/e/add/wZwpmL1MTM5gTM5cDMyMzM1UTM1QDN5MjM5ADMwAjMwUzL3AzL2czLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
解:
![輪換式](/img/7/ee7/wZwpmLwQDMyAjNwgTOwMzM1UTM1QDN5MjM5ADMwAjMwUzL4kzL0gzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
∴原式
![輪換式](/img/0/ab6/wZwpmL1YDO4YTOycDMxMzM1UTM1QDN5MjM5ADMwAjMwUzL3AzLyIzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![輪換式](/img/f/d02/wZwpmL3YTOyITO4gjNwMzM1UTM1QDN5MjM5ADMwAjMwUzL4YzLzEzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![輪換式](/img/2/316/wZwpmL0IjM2UDO1ADMyMzM1UTM1QDN5MjM5ADMwAjMwUzLwAzL4EzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![輪換式](/img/5/4f9/wZwpmLwgTOzYjNzkjNwMzM1UTM1QDN5MjM5ADMwAjMwUzL5YzL4EzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
例2:分解因式
![輪換式](/img/d/8e3/wZwpmL1AzMxEzMzkjNwMzM1UTM1QDN5MjM5ADMwAjMwUzL5YzL3czLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![輪換式](/img/7/44a/wZwpmLxQTO1cDOwkDOxMzM1UTM1QDN5MjM5ADMwAjMwUzL5gzLxgzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![輪換式](/img/8/321/wZwpmLzMjN3QDO0EzMxMzM1UTM1QDN5MjM5ADMwAjMwUzLxMzL2UzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![輪換式](/img/f/37f/wZwpmLxEDO5AzN1kDOxMzM1UTM1QDN5MjM5ADMwAjMwUzL5gzL0YzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![輪換式](/img/f/31b/wZwpmL1MzNykDOzczMxMzM1UTM1QDN5MjM5ADMwAjMwUzL3MzL1gzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![輪換式](/img/8/023/wZwpmLyETOxUTMyQzM2EzM1UTM1QDN5MjM5ADMwAjMwUzL0MzL2EzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![輪換式](/img/4/e8a/wZwpmL3UjNxETMxIDN2EzM1UTM1QDN5MjM5ADMwAjMwUzLyQzL2gzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![輪換式](/img/d/5bf/wZwpmL0gjNwgzMyEjMwEDN0UTMyITNykTO0EDMwAjMwUzLxIzL0UzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![輪換式](/img/f/37f/wZwpmLxEDO5AzN1kDOxMzM1UTM1QDN5MjM5ADMwAjMwUzL5gzL0YzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![輪換式](/img/9/956/wZwpmL1UDO3AjNzQTMxMzM1UTM1QDN5MjM5ADMwAjMwUzL0EzL1MzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![輪換式](/img/8/af8/wZwpmLxgjN5kDN5EzMwEDN0UTMyITNykTO0EDMwAjMwUzLxMzL1EzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![輪換式](/img/f/37f/wZwpmLxEDO5AzN1kDOxMzM1UTM1QDN5MjM5ADMwAjMwUzL5gzL0YzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![輪換式](/img/d/e96/wZwpmL4ATOzMTN1YzMxMzM1UTM1QDN5MjM5ADMwAjMwUzL2MzLyMzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
此題中若將式中的b換成a,c換成b,a換成c,即為 ,原式不變,這類多項式稱為關於a、b、c的輪換對稱式,輪換對稱式的因式分解,用因式定理及待定係數法比較簡單。下面先粗略介紹一下因式定理,為了敘述方便先引入符號 , 如對一元多項式 可記作 , 即表示當 時多項式的值,如 時,多項式 的值為 ;當 時多項式 的值為
![輪換式](/img/4/e8a/wZwpmL3UjNxETMxIDN2EzM1UTM1QDN5MjM5ADMwAjMwUzLyQzL2gzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![輪換式](/img/f/22b/wZwpmLzUTO4cDMxgDOwMzM1UTM1QDN5MjM5ADMwAjMwUzL4gzL2gzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![輪換式](/img/2/c32/wZwpmL2QjM2gDO1kTNwMzM1UTM1QDN5MjM5ADMwAjMwUzL5UzL1YzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![輪換式](/img/f/22b/wZwpmLzUTO4cDMxgDOwMzM1UTM1QDN5MjM5ADMwAjMwUzL4gzL2gzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![輪換式](/img/5/198/wZwpmL1MDOycDMwczNxMzM1UTM1QDN5MjM5ADMwAjMwUzL3czL1MzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![輪換式](/img/8/2d8/wZwpmLyQTM4gTM0YzNwMzM1UTM1QDN5MjM5ADMwAjMwUzL2czLxEzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
因式定理:如果 時多項式 的值為零,即 則 能被 整除(即含有 的因式)。
![輪換式](/img/f/31b/wZwpmL1MzNykDOzczMxMzM1UTM1QDN5MjM5ADMwAjMwUzL3MzL1gzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![輪換式](/img/8/af8/wZwpmLxgjN5kDN5EzMwEDN0UTMyITNykTO0EDMwAjMwUzLxMzL1EzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![輪換式](/img/d/8cd/wZwpmL1ADNycDN1QTOwMzM1UTM1QDN5MjM5ADMwAjMwUzL0kzLzMzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![輪換式](/img/f/22b/wZwpmLzUTO4cDMxgDOwMzM1UTM1QDN5MjM5ADMwAjMwUzL4gzL2gzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![輪換式](/img/7/d9c/wZwpmLwYTN5QjN1ITOwMzM1UTM1QDN5MjM5ADMwAjMwUzLykzL2czLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![輪換式](/img/2/b42/wZwpmLwQDM2UTNzITOwMzM1UTM1QDN5MjM5ADMwAjMwUzLykzL4gzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
如多項式 ,當 時, 即 含有 的因式,事實上 .
![輪換式](/img/2/14c/wZwpmL3YDOwMjN1IzNwMzM1UTM1QDN5MjM5ADMwAjMwUzLyczLyUzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![輪換式](/img/f/870/wZwpmLwUDO2EDNyQzNxMzM1UTM1QDN5MjM5ADMwAjMwUzL0czL3IzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
對於一般的形式: ,若 ,則
![輪換式](/img/6/59d/wZwpmL3UDM3EDO3cTOwMzM1UTM1QDN5MjM5ADMwAjMwUzL3kzLxIzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![輪換式](/img/8/e1e/wZwpmL3ETMyQTOwkjNwMzM1UTM1QDN5MjM5ADMwAjMwUzL5YzLxEzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![輪換式](/img/4/9b8/wZwpmL4gjM0IjM5gjNwMzM1UTM1QDN5MjM5ADMwAjMwUzL4YzLyczLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
由於 (其中 | 表示整除)
![輪換式](/img/2/6e3/wZwpmL1QDM5YDM0ITOwMzM1UTM1QDN5MjM5ADMwAjMwUzLykzLyIzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
∴
由以上分析可知:對於多元多項式,在使用因式定理時可以確定一個主元,而將其它的元看成確定的數來處理.
我們利用因式定理來解例2:
![輪換式](/img/c/1d6/wZwpmL1gzN3EzMyEzNwMzM1UTM1QDN5MjM5ADMwAjMwUzLxczL2UzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![輪換式](/img/4/3fd/wZwpmL4MDO2EDNyQzNxMzM1UTM1QDN5MjM5ADMwAjMwUzL0czL3EzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![輪換式](/img/0/86f/wZwpmL1MTN5EjN3MzNwMzM1UTM1QDN5MjM5ADMwAjMwUzLzczL2QzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![輪換式](/img/a/92a/wZwpmL1ETO0UzN5IzNxMzM1UTM1QDN5MjM5ADMwAjMwUzLyczL4MzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![輪換式](/img/b/dae/wZwpmLwcTM4UzMwUTMxMzM1UTM1QDN5MjM5ADMwAjMwUzL1EzLzgzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
解 這是一個含有a、b、c三個字母的三次多項式,現以a為主元,設 ,易知當a=b和a=c時,都有 ,故a-b和a-c是多項式的因式,而視b為主元時,同理可知b-c也是多項式的因式,而三次多項式至多有三個因式故可設 ,其中k為待定係數,令 可得
![輪換式](/img/4/214/wZwpmLyATNzMTM1kTNwMzM1UTM1QDN5MjM5ADMwAjMwUzL5UzLzEzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
∴
![輪換式](/img/3/284/wZwpmL3MzM0QjN5UzMxMzM1UTM1QDN5MjM5ADMwAjMwUzL1MzL0QzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
例3分解因式
![輪換式](/img/b/b2b/wZwpmLwMTM5EzMycjNxMzM1UTM1QDN5MjM5ADMwAjMwUzL3YzL4gzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![輪換式](/img/7/99b/wZwpmL0UjN0cDM2gzNwMzM1UTM1QDN5MjM5ADMwAjMwUzL4czL2gzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![輪換式](/img/f/b5c/wZwpmL4gTN1YjM3gjNxMzM1UTM1QDN5MjM5ADMwAjMwUzL4YzL3AzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![輪換式](/img/0/d37/wZwpmLwgzM1EjMzUTMxMzM1UTM1QDN5MjM5ADMwAjMwUzL1EzLzEzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![輪換式](/img/b/dae/wZwpmLwcTM4UzMwUTMxMzM1UTM1QDN5MjM5ADMwAjMwUzL1EzLzgzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
分析: 這是一個關於a、b、c的四次齊次輪換多項式,可用因式定理分解,易知 是多項式的三個因式,而四次多項式還有一個因式,由輪換對稱性可知這個一次因式應是 ,故可設 (其中k為待定係數),取 可得 ,所以
![輪換式](/img/0/2f2/wZwpmLyEDMxkzM0AzNwMzM1UTM1QDN5MjM5ADMwAjMwUzLwczLxAzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
原式
輪換式的形式
![輪換式](/img/f/3fa/wZwpmL3gTNyUDM2QDOwMzM1UTM1QDN5MjM5ADMwAjMwUzL0gzL0gzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
一次齊次的輪換式形如:
![輪換式](/img/f/274/wZwpmL4UzMycDMwgTMyMzM1UTM1QDN5MjM5ADMwAjMwUzL4EzL1IzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
二次齊次的輪換式形如:
![輪換式](/img/e/750/wZwpmL0YjMxkDN3kTNxMzM1UTM1QDN5MjM5ADMwAjMwUzL5UzL4YzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
三次齊次的輪換式形如:
其中的 l, m, n, k 是待定常數.
齊次與非齊次
![輪換式](/img/c/312/wZwpmLwADMzgzN3MDOwMzM1UTM1QDN5MjM5ADMwAjMwUzLzgzL0UzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
分解因式
![輪換式](/img/0/dce/wZwpmL2EDM2kjM0YTMxMzM1UTM1QDN5MjM5ADMwAjMwUzL2EzL4EzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
易知其有因式 。因為原式是五次齊次輪換式,所以還缺一個二次齊次輪換式。不妨設
![輪換式](/img/e/9bc/wZwpmL0AzM3czN0ITOwMzM1UTM1QDN5MjM5ADMwAjMwUzLykzLzAzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![輪換式](/img/b/b5b/wZwpmL2gTO5cjM5YzNxMzM1UTM1QDN5MjM5ADMwAjMwUzL2czL3gzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![輪換式](/img/c/25f/wZwpmL1UDO0EjNzcTOwMzM1UTM1QDN5MjM5ADMwAjMwUzL3kzLwEzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
令 可得
![輪換式](/img/4/e75/wZwpmL4IzNxMDN3ITOxMzM1UTM1QDN5MjM5ADMwAjMwUzLykzLyIzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![輪換式](/img/5/c55/wZwpmL2IjNzUDN4ITOwMzM1UTM1QDN5MjM5ADMwAjMwUzLykzL2czLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
令 可得
![輪換式](/img/c/223/wZwpmL2IzNxczN4ETOxMzM1UTM1QDN5MjM5ADMwAjMwUzLxkzL3EzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
於是可得 。這就給出了所要的因式分解。
![輪換式](/img/9/1a7/wZwpmL3IDO3MjNzgjMxMzM1UTM1QDN5MjM5ADMwAjMwUzL4IzLxQzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
分解因式:
![輪換式](/img/4/c6d/wZwpmLzUDN5MDMxcTMxMzM1UTM1QDN5MjM5ADMwAjMwUzL3EzL1AzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![輪換式](/img/e/d8f/wZwpmL1UTM2cTOyMjNwMzM1UTM1QDN5MjM5ADMwAjMwUzLzYzL4EzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
記 , 則此題就變為上一例題。最後結果為:
![輪換式](/img/b/474/wZwpmLyYjM5ITM5ITOwMzM1UTM1QDN5MjM5ADMwAjMwUzLykzL3gzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
一個有用的公式
![輪換式](/img/7/557/wZwpmLzAzM3gDOxMTOwMzM1UTM1QDN5MjM5ADMwAjMwUzLzkzLzYzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
分解因式:
![輪換式](/img/9/bd6/wZwpmL4ADOzMzMzMjNwMzM1UTM1QDN5MjM5ADMwAjMwUzLzYzLxczLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![輪換式](/img/8/5f4/wZwpmLwcTO5YzN4EzNxMzM1UTM1QDN5MjM5ADMwAjMwUzLxczLzIzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
當 時,原式為 0,所以原式有因式 。再者,原式是三次齊次輪換式,所以我們還缺一個二次齊次輪換式因式。 不妨設
![輪換式](/img/4/68c/wZwpmL4MDNxADO4gzNxMzM1UTM1QDN5MjM5ADMwAjMwUzL4czL1QzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![輪換式](/img/a/034/wZwpmLwcDOyEzNyMTMzEDN0UTMyITNykTO0EDMwAjMwUzLzEzLwMzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![輪換式](/img/6/c42/wZwpmL3EDNykjMxAzNwMzM1UTM1QDN5MjM5ADMwAjMwUzLwczLzgzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![輪換式](/img/5/a48/wZwpmL3gDM1ETM3cjMwgDM1UTM1QDN5MjM5ADMwAjMwUzL3IzL0EzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![輪換式](/img/7/909/wZwpmL3gDM2gDO5MzMxMzM1UTM1QDN5MjM5ADMwAjMwUzLzMzL2EzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
比較兩邊 的係數可得 。比較 的係數可得 ,於是:
![輪換式](/img/1/2f2/wZwpmL4cTN4gDM5MTOwMzM1UTM1QDN5MjM5ADMwAjMwUzLzkzL1EzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
有時候我們也把它寫為
![輪換式](/img/5/a63/wZwpmL0gzN2AzMyczNwMzM1UTM1QDN5MjM5ADMwAjMwUzL3czLzIzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)