簡介
![微分法](/img/8/3ed/wZwpmLxADN2QTO4MzNxADN0UTMyITNykTO0EDMwAjMwUzLzczL0YzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/9/d27/wZwpmL2ATN5IzM1gjNwMzM1UTM1QDN5MjM5ADMwAjMwUzL4YzLzMzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
在古典的微積分學中,微分被定義為變化量的線性部分,在現代的定義中,微分被定義為將自變數的改變數 映射到變化量的線性部分的線性映射 。這個映射也被稱為切映射。給定的函式在一點的微分如果存在,就一定是唯一的。
![微分法](/img/8/778/wZwpmL2MzN5UzM2MzNwIDN0UTMyITNykTO0EDMwAjMwUzLzczL4MzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![微分法](/img/1/036/wZwpmL3QzM5YjNzMjM0EDN0UTMyITNykTO0EDMwAjMwUzLzIzL3EzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![微分法](/img/8/3ed/wZwpmLxADN2QTO4MzNxADN0UTMyITNykTO0EDMwAjMwUzLzczL0YzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/8/3ed/wZwpmLxADN2QTO4MzNxADN0UTMyITNykTO0EDMwAjMwUzLzczL0YzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/8/3ed/wZwpmLxADN2QTO4MzNxADN0UTMyITNykTO0EDMwAjMwUzLzczL0YzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/8/3ed/wZwpmLxADN2QTO4MzNxADN0UTMyITNykTO0EDMwAjMwUzLzczL0YzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/8/778/wZwpmL2MzN5UzM2MzNwIDN0UTMyITNykTO0EDMwAjMwUzLzczL4MzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![微分法](/img/1/036/wZwpmL3QzM5YjNzMjM0EDN0UTMyITNykTO0EDMwAjMwUzLzIzL3EzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![微分法](/img/8/3ed/wZwpmLxADN2QTO4MzNxADN0UTMyITNykTO0EDMwAjMwUzLzczL0YzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/8/3ed/wZwpmLxADN2QTO4MzNxADN0UTMyITNykTO0EDMwAjMwUzLzczL0YzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/8/3ed/wZwpmLxADN2QTO4MzNxADN0UTMyITNykTO0EDMwAjMwUzLzczL0YzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/8/3ed/wZwpmLxADN2QTO4MzNxADN0UTMyITNykTO0EDMwAjMwUzLzczL0YzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/1/036/wZwpmL3QzM5YjNzMjM0EDN0UTMyITNykTO0EDMwAjMwUzLzIzL3EzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![微分法](/img/8/5ec/wZwpmLzADMwEjM4ITOwMzM1UTM1QDN5MjM5ADMwAjMwUzLykzL4YzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![微分法](/img/1/709/wZwpmLxMDO5UzN4gjNwMzM1UTM1QDN5MjM5ADMwAjMwUzL4YzLxEzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
在數學中,微分是對函式的局部變化率的一種線性描 述。微分可以近似地描述當函式自變數的取值作足夠小的改變時,函式的值是怎樣改變的。當某些函式 的自變數 有一個微小的改變 時,函式的變化可以分解為兩個部分。一個部分是線性部分:在一維情況下,它正比於自變數的變化量 ,可以表示成 和一個與 無關,只與函式 及 有關的量的乘積;在更廣泛的情況下,它是一個線性映射作用在 上的值。另一部分是比 更高階的無窮小,也就是說除以 後仍然會趨於零。當改變數很小時,第二部分可以忽略不計,函式的變化量約等於第一部分,也就是函式在 處的 微分,記作 或 。如果一個函式在某處具有以上的性質,就稱此函式在該點可微 。
不是所有的函式的變化量都可以分為以上提到的兩個部分。若函式在某一點無法做到可微,便稱函式在該點不可微。
定義
微分法定義如下:
![微分法](/img/7/207/wZwpmLwcjM4QDO2kzNwMzM1UTM1QDN5MjM5ADMwAjMwUzL5czL3YzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![微分法](/img/a/6f2/wZwpmL3UjN1QTN1MzNwIDN0UTMyITNykTO0EDMwAjMwUzLzczLwQzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/a/6f2/wZwpmL3UjN1QTN1MzNwIDN0UTMyITNykTO0EDMwAjMwUzLzczLwQzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/d/58f/wZwpmLyEjN4ITNyMTMzEDN0UTMyITNykTO0EDMwAjMwUzLzEzLxczLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![微分法](/img/d/58f/wZwpmLyEjN4ITNyMTMzEDN0UTMyITNykTO0EDMwAjMwUzLzEzLxczLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![微分法](/img/2/d0e/wZwpmLzIjN0kDN4ATMyMzM1UTM1QDN5MjM5ADMwAjMwUzLwEzL3czLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![微分法](/img/2/133/wZwpmL3cDO0ADOwkDO0ATN0UTMyITNykTO0EDMwAjMwUzL5gzLzIzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/c/8ac/wZwpmLyQTO0kDM0gTOwMzM1UTM1QDN5MjM5ADMwAjMwUzL4kzLyUzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![微分法](/img/c/c8f/wZwpmL3gTMycjM4QTOwADN0UTMyITNykTO0EDMwAjMwUzL0kzL3MzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![微分法](/img/8/61a/wZwpmL1ETN0kzMwYjMzEzM1UTM1QDN5MjM5ADMwAjMwUzL2IzL1QzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/1/802/wZwpmLxEzM3cTN1kDOxMzM1UTM1QDN5MjM5ADMwAjMwUzL5gzLxQzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![微分法](/img/b/cd4/wZwpmLxcTNyUjNwMDO4EDN0UTMyITNykTO0EDMwAjMwUzLzgzLzIzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![微分法](/img/d/58f/wZwpmLyEjN4ITNyMTMzEDN0UTMyITNykTO0EDMwAjMwUzLzEzLxczLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![微分法](/img/e/d4c/wZwpmL2YzN1MTOxgjNwMzM1UTM1QDN5MjM5ADMwAjMwUzL4YzLzEzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![微分法](/img/d/58f/wZwpmLyEjN4ITNyMTMzEDN0UTMyITNykTO0EDMwAjMwUzLzEzLxczLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![微分法](/img/8/61a/wZwpmL1ETN0kzMwYjMzEzM1UTM1QDN5MjM5ADMwAjMwUzL2IzL1QzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/8/12f/wZwpmLwADMwUTM2AjMxMzM1UTM1QDN5MjM5ADMwAjMwUzLwIzL1gzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![微分法](/img/0/db8/wZwpmL3cDN4ETO1kTOxMzM1UTM1QDN5MjM5ADMwAjMwUzL5kzL0QzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![微分法](/img/8/12f/wZwpmLwADMwUTM2AjMxMzM1UTM1QDN5MjM5ADMwAjMwUzLwIzL1gzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![微分法](/img/d/19d/wZwpmL1MzNxcDM4EzNwMzM1UTM1QDN5MjM5ADMwAjMwUzLxczL1IzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
設函式 在某區間 內有定義。對於 內一點 ,當 變動到附近的 (也在此區間內)時,如果函式的增量 可表示為 (其中 是不依賴於 的常數),而 是比高階的無窮小,那么稱函式 在點 是可微的,且 稱作函式在點 相應於自變數增量 的微分,記作 ,即 , 是 的線性主部。
![微分法](/img/1/036/wZwpmL3QzM5YjNzMjM0EDN0UTMyITNykTO0EDMwAjMwUzLzIzL3EzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![微分法](/img/8/61a/wZwpmL1ETN0kzMwYjMzEzM1UTM1QDN5MjM5ADMwAjMwUzL2IzL1QzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/8/3e5/wZwpmLyADN4cTN2MzNwIDN0UTMyITNykTO0EDMwAjMwUzLzczL3gzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/2/c6f/wZwpmLxYzNxQjN5YTMxMzM1UTM1QDN5MjM5ADMwAjMwUzL2EzL2QzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
通常把自變數 的增量 稱為自變數的微分,記作 ,即 。
![圖1.微分法](/img/9/309/wZwpmLxgDO2MDM1ETOxMzM1UTM1QDN5MjM5ADMwAjMwUzLxkzL4YzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![微分法](/img/8/12f/wZwpmLwADMwUTM2AjMxMzM1UTM1QDN5MjM5ADMwAjMwUzLwIzL1gzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![微分法](/img/d/19d/wZwpmL1MzNxcDM4EzNwMzM1UTM1QDN5MjM5ADMwAjMwUzLxczL1IzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
(函式在一點的微分,其中紅線部分是微分量 ,而加上灰線部分後是實際的改變數 。)
幾何意義
![微分法](/img/8/61a/wZwpmL1ETN0kzMwYjMzEzM1UTM1QDN5MjM5ADMwAjMwUzL2IzL1QzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/6/fe7/wZwpmL1EzNwUDO2kzNwMzM1UTM1QDN5MjM5ADMwAjMwUzL5czL0UzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![微分法](/img/b/848/wZwpmL1YDNwYzMwMzMzIDN0UTMyITNykTO0EDMwAjMwUzLzMzL4QzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![微分法](/img/d/19d/wZwpmL1MzNxcDM4EzNwMzM1UTM1QDN5MjM5ADMwAjMwUzLxczL1IzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![微分法](/img/b/848/wZwpmL1YDNwYzMwMzMzIDN0UTMyITNykTO0EDMwAjMwUzLzMzL4QzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![微分法](/img/8/61a/wZwpmL1ETN0kzMwYjMzEzM1UTM1QDN5MjM5ADMwAjMwUzL2IzL1QzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/8/12f/wZwpmLwADMwUTM2AjMxMzM1UTM1QDN5MjM5ADMwAjMwUzLwIzL1gzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![微分法](/img/b/848/wZwpmL1YDNwYzMwMzMzIDN0UTMyITNykTO0EDMwAjMwUzLzMzL4QzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![微分法](/img/8/61a/wZwpmL1ETN0kzMwYjMzEzM1UTM1QDN5MjM5ADMwAjMwUzL2IzL1QzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/e/9d3/wZwpmL4UTO3YTN3kDMxMzM1UTM1QDN5MjM5ADMwAjMwUzL5AzLxAzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![微分法](/img/6/b7b/wZwpmL0YTM0kDO0ETMyMzM1UTM1QDN5MjM5ADMwAjMwUzLxEzLwAzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![微分法](/img/0/728/wZwpmLwcTN2YjM2ADOxMzM1UTM1QDN5MjM5ADMwAjMwUzLwgzL4IzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![微分法](/img/b/848/wZwpmL1YDNwYzMwMzMzIDN0UTMyITNykTO0EDMwAjMwUzLzMzL4QzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
設 是曲線 上的點 在橫坐標上的增量,是曲線在點 對應 在縱坐標上的增量, 是曲線在點 的切線對應 在縱坐標上的增量。當 很小時, 比 要小得多(高階無窮小),因此在點 附近,我們可以用切線段來近似代替曲線段 。
微分法則
![微分法](/img/a/ed1/wZwpmL4ITO2IzMyQTM5czN0UTMyITNykTO0EDMwAjMwUzL0EzLwEzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![微分法](/img/c/e70/wZwpmL1ITO3IDO3AjMyADN0UTMyITNykTO0EDMwAjMwUzLwIzLyczLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
和求導一樣,微分有類似的法則,例如,如果設函式 、 可微,那么:
![微分法](/img/4/c98/wZwpmLwQjM3czMykTOwMzM1UTM1QDN5MjM5ADMwAjMwUzL5kzLxQzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
1)
![微分法](/img/3/bbc/wZwpmL0IjM0cTN5MTOxMzM1UTM1QDN5MjM5ADMwAjMwUzLzkzL3YzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
2)
![微分法](/img/a/1b5/wZwpmLwYDMzQjNyYTMxMzM1UTM1QDN5MjM5ADMwAjMwUzL2EzL2QzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
3) ,
![微分法](/img/9/9f8/wZwpmLwcTOxQTO1cDMxMzM1UTM1QDN5MjM5ADMwAjMwUzL3AzL3QzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![微分法](/img/1/644/wZwpmLyYTMyMzN5EzNwMzM1UTM1QDN5MjM5ADMwAjMwUzLxczL1czLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
4)若函式 可導,那么 。
微分法與微分形式
![微分法](/img/1/036/wZwpmL3QzM5YjNzMjM0EDN0UTMyITNykTO0EDMwAjMwUzLzIzL3EzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![微分法](/img/a/14c/wZwpmL4cTN5IzM1gjNwMzM1UTM1QDN5MjM5ADMwAjMwUzL4YzLzAzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![微分法](/img/b/ccc/wZwpmL0ETN0ETO1UTMxMzM1UTM1QDN5MjM5ADMwAjMwUzL1EzLzgzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![微分法](/img/d/64f/wZwpmLwcDO2gTN4UTOxMzM1UTM1QDN5MjM5ADMwAjMwUzL1kzL1AzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
如果說微分是導數的一種推廣,那么微分形式則是對於微分函式的再推廣。微分函式對每個點 給出一個近似描述函式性質的線性映射 ,而微分形式對區域 內的每一點給出一個從該點的切空間映射到值域的斜對稱形式: 。在坐標記法下,可以寫成:
![微分法](/img/e/a83/wZwpmLwETOxYjNyMTOwMzM1UTM1QDN5MjM5ADMwAjMwUzLzkzLyczLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![微分法](/img/7/2c5/wZwpmLxADM5UzN4gTNxMzM1UTM1QDN5MjM5ADMwAjMwUzL4UzLzgzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![微分法](/img/6/281/wZwpmL0MDOxIjMxMTMzEDN0UTMyITNykTO0EDMwAjMwUzLzEzL0czLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![微分法](/img/c/e70/wZwpmL1ITO3IDO3AjMyADN0UTMyITNykTO0EDMwAjMwUzLwIzLyczLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![微分法](/img/6/281/wZwpmL0MDOxIjMxMTMzEDN0UTMyITNykTO0EDMwAjMwUzLzEzL0czLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![微分法](/img/9/c16/wZwpmL2IDN4gDOyczNxMzM1UTM1QDN5MjM5ADMwAjMwUzL3czLzIzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![微分法](/img/1/285/wZwpmLygDN4QzMycjNxMzM1UTM1QDN5MjM5ADMwAjMwUzL3YzLyUzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
其中的是-射影運算元,也就是說將一個向量射到它的第個分量的映射。而是滿足:
![微分法](/img/1/76e/wZwpmLygzNwADO1MTOwMzM1UTM1QDN5MjM5ADMwAjMwUzLzkzLxUzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
的 k-形式。
![微分法](/img/8/778/wZwpmL2MzN5UzM2MzNwIDN0UTMyITNykTO0EDMwAjMwUzLzczL4MzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![微分法](/img/5/b84/wZwpmL4cTO0kjM5EzNwMzM1UTM1QDN5MjM5ADMwAjMwUzLxczLyczLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![微分法](/img/4/a34/wZwpmL4czM5YTM5cTO4kzM0UTMyITNykTO0EDMwAjMwUzL3kzL4czLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![微分法](/img/9/d27/wZwpmL2ATN5IzM1gjNwMzM1UTM1QDN5MjM5ADMwAjMwUzL4YzLzMzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
特別地,當是一個從映射到的函式時,可以將寫作:
![微分法](/img/9/7ee/wZwpmL1MzNwIDN5kjNwMzM1UTM1QDN5MjM5ADMwAjMwUzL5YzLxIzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
正是上面公式的一個特例。