定義
![積分上限函式](/img/c/e67/wZwpmL2gjM3AjN3EjNxMzM1UTM1QDN5MjM5ADMwAjMwUzLxYzL1MzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![積分上限函式](/img/d/159/wZwpmLzYjNxgTN1QDOwMzM1UTM1QDN5MjM5ADMwAjMwUzL0gzLyIzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![積分上限函式](/img/3/78e/wZwpmLyMDNzQDM5EjNxADN0UTMyITNykTO0EDMwAjMwUzLxYzL2AzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![積分上限函式](/img/0/37d/wZwpmL0AzMwAjN0QDOwMzM1UTM1QDN5MjM5ADMwAjMwUzL0gzLzYzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
設函式f(x)在區間[a,b]上可積,且對任意 在[a,x]上也可積,稱變上限定積分 為 的積分上限函式,記為 即
![積分上限函式](/img/4/e78/wZwpmLxcDO0EDOyITOxMzM1UTM1QDN5MjM5ADMwAjMwUzLykzLxczLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![積分上限函式](/img/1/d08/wZwpmL2EjN0IDM1AzNwMzM1UTM1QDN5MjM5ADMwAjMwUzLwczL1QzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![積分上限函式](/img/d/fe6/wZwpmL1QzMxgjMxkTNxMzM1UTM1QDN5MjM5ADMwAjMwUzL5UzLyAzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
當 時, 在幾何上表示為右側鄰邊可以變動的曲邊梯形的面積(圖1中的陰影部分) 。
![圖1](/img/2/748/wZwpmLwATM5UzM0YTNxMzM1UTM1QDN5MjM5ADMwAjMwUzL2UzLyczLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
定理
![積分上限函式](/img/3/78e/wZwpmLyMDNzQDM5EjNxADN0UTMyITNykTO0EDMwAjMwUzLxYzL2AzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
設函式 在區間[a,b]上連續,則積分上限函式
![積分上限函式](/img/a/3f6/wZwpmL0YTN3EjNzQTOxMzM1UTM1QDN5MjM5ADMwAjMwUzL0kzL0QzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
在[a,b]上可導,並且
![積分上限函式](/img/0/02d/wZwpmLxQDOyATMwcTOwMzM1UTM1QDN5MjM5ADMwAjMwUzL3kzLxEzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![積分上限函式](/img/8/2dd/wZwpmLyQzMzYTN4EzNwMzM1UTM1QDN5MjM5ADMwAjMwUzLxczL3gzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![積分上限函式](/img/a/6e4/wZwpmL3QzM1YTO4ETNyMzM1UTM1QDN5MjM5ADMwAjMwUzLxUzLzQzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![積分上限函式](/img/6/6f9/wZwpmL4IjNyMjN0QzNxMzM1UTM1QDN5MjM5ADMwAjMwUzL0czLxYzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![積分上限函式](/img/d/fe6/wZwpmL1QzMxgjMxkTNxMzM1UTM1QDN5MjM5ADMwAjMwUzL5UzLyAzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
證明: 對於任意給定的 給x以增量 使得 由 的定義及定積分對區間的可加性,有
![積分上限函式](/img/b/038/wZwpmLwQDMzgTM0cjNxMzM1UTM1QDN5MjM5ADMwAjMwUzL3YzLwgzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![積分上限函式](/img/6/fe6/wZwpmLxcTMxYjN4YTMxMzM1UTM1QDN5MjM5ADMwAjMwUzL2EzLxIzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
再由 定積分中值定理,得
![積分上限函式](/img/9/7fc/wZwpmL3cTOwQzMzcDMxMzM1UTM1QDN5MjM5ADMwAjMwUzL3AzL0MzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![積分上限函式](/img/9/cc2/wZwpmLzATN2MTMzIjM2EzM1UTM1QDN5MjM5ADMwAjMwUzLyIzL1IzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![積分上限函式](/img/1/036/wZwpmL3QzM5YjNzMjM0EDN0UTMyITNykTO0EDMwAjMwUzLzIzL3EzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![積分上限函式](/img/5/8ad/wZwpmL0cTM0ITO3kTNxMzM1UTM1QDN5MjM5ADMwAjMwUzL5UzLygzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
其中, 在 和 之間。
![積分上限函式](/img/c/5cc/wZwpmLwETN3UjN2EDMyMzM1UTM1QDN5MjM5ADMwAjMwUzLxAzL2czLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![積分上限函式](/img/d/6cc/wZwpmLyATMxUzN3AzNwMzM1UTM1QDN5MjM5ADMwAjMwUzLwczL4QzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![積分上限函式](/img/4/5f0/wZwpmL3ATO1YzMxgjMxMzM1UTM1QDN5MjM5ADMwAjMwUzL4IzL4MzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![積分上限函式](/img/3/78e/wZwpmLyMDNzQDM5EjNxADN0UTMyITNykTO0EDMwAjMwUzLxYzL2AzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
令 則 從而 由 的連續性,得
![積分上限函式](/img/3/bbe/wZwpmL0cjN0UzN3ITOwMzM1UTM1QDN5MjM5ADMwAjMwUzLykzL3czLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
根據導數定義,得
![積分上限函式](/img/8/9b8/wZwpmL1MzMwUTN5kTNwMzM1UTM1QDN5MjM5ADMwAjMwUzL5UzLyEzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
即
![積分上限函式](/img/e/e43/wZwpmLzgjN0IzMwczMxMzM1UTM1QDN5MjM5ADMwAjMwUzL3MzL2czLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
證畢。
![積分上限函式](/img/6/3f0/wZwpmL1MDO2EDN3ITOxMzM1UTM1QDN5MjM5ADMwAjMwUzLykzL2IzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![積分上限函式](/img/3/78e/wZwpmLyMDNzQDM5EjNxADN0UTMyITNykTO0EDMwAjMwUzLxYzL2AzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
這個定理說明,任何連續函式都有原函式存在,且積分上限函式 就是在[a,b] 上的一個原函式。上述定理也叫做 原函式存在定理 。