定義
複數的模與輻角是複數三角形式表示的兩個基本元素,複數所對應的向量長度稱為複數的幅值,該向量與實軸正方向的夾角為複數的輻角。輻角的大小有無窮多,但是輻角主值唯一確定。
![輻角主值](/img/6/018/wZwpmLyczNwQTM2YTMxYjN1UTM1QDN5MjM5ADMwAjMwUzL2EzL1czLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
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![輻角主值](/img/2/047/wZwpmL4YzMwUTN3ADO3EDN0UTMyITNykTO0EDMwAjMwUzLwgzLygzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![輻角主值](/img/2/047/wZwpmL4YzMwUTN3ADO3EDN0UTMyITNykTO0EDMwAjMwUzLwgzLygzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
由於一個複數 可以由有序實數對 唯一確定,而有序實數對與平面直角坐標系 中的點一一對應,因此可以用坐標為 的點 來表示該複數,此時 軸上的點與實數對應,稱 軸為實軸, 軸上的點(除原點外)與純虛數對應,稱 軸為虛軸,像這樣表示複數的平面稱為複平面。
![輻角主值](/img/0/46a/wZwpmL3gTN3ATMzMzM3UzM1UTM1QDN5MjM5ADMwAjMwUzLzMzLwczLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
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![輻角主值](/img/1/036/wZwpmL3QzM5YjNzMjM0EDN0UTMyITNykTO0EDMwAjMwUzLzIzL3EzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
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複數 還可以用向量 來表示, 與 分別是向量在 軸與 軸上的投影。這樣,複數 就與平面上的向量 建立了一一對應的關係。
![輻角主值](/img/8/0d2/wZwpmLzUjM2ATO4UDM5IDN0UTMyITNykTO0EDMwAjMwUzL1AzLwAzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
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向量 的長度稱為複數 的模或絕對值,記作 ,於是
![輻角主值](/img/6/f64/wZwpmL2EzN0cDM5ETOwcTN1UTM1QDN5MjM5ADMwAjMwUzLxkzLxEzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
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![輻角主值](/img/d/348/wZwpmL1ATO4gTOwkzN4MTN0UTMyITNykTO0EDMwAjMwUzL5czLxczLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
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當點 不是原點,即複數 時,向量 與 軸正向的夾角稱為複數 的輻角,記作 。輻角的符號規定為:由正實軸依反時針方向轉到 為正,依順時針方向轉到 為負。
![輻角主值](/img/d/348/wZwpmL1ATO4gTOwkzN4MTN0UTMyITNykTO0EDMwAjMwUzL5czLxczLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
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顯然一個非零複數 的輻角有無窮多個值,它們相差 的整數倍,但中只有一個值 滿足條件 ,稱 為複數 的主輻角,記為 ,於是
![輻角主值](/img/f/d70/wZwpmL4ATO4EzNxEzMxYjN1UTM1QDN5MjM5ADMwAjMwUzLxMzLxQzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![輻角主值](/img/6/d8c/wZwpmLxUzN4AzMxIjM2EzM1UTM1QDN5MjM5ADMwAjMwUzLyIzL2QzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
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當 時, 的輻角沒有意義。
輻角主值的計算
![輻角主值](/img/0/46a/wZwpmL3gTN3ATMzMzM3UzM1UTM1QDN5MjM5ADMwAjMwUzLzMzLwczLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![輻角主值](/img/a/c7f/wZwpmL3ITM1gjMxUjMxYjN1UTM1QDN5MjM5ADMwAjMwUzL1IzLyEzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![輻角主值](/img/f/648/wZwpmLwQDOxMDM3QjMxYjN1UTM1QDN5MjM5ADMwAjMwUzL0IzL0gzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
複數 的主輔角與反正切的主值有以下關係:
![輻角主值](/img/c/f57/wZwpmL3IzM1gTN4QDNxYjN1UTM1QDN5MjM5ADMwAjMwUzL0QzL4MzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)