基本內容
直線的基本性質(公理): (1)經過兩點有一條直線,並且只有一條直線。(2)兩條直線相交,只有一個交點。因為直線是不定義的名詞,對直線概念的理解往往靠上述的基本性質。
套用舉例
![直線公理](/img/3/c48/wZwpmL3YDNwkTN4ITO2UzM1UTM1QDN5MjM5ADMwAjMwUzLykzL0IzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![直線公理](/img/7/650/wZwpmL4IzN3gjMzATMzEzM1UTM1QDN5MjM5ADMwAjMwUzLwEzL2gzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
【例1】已知點 ,求 的面積。
![圖1](/img/d/c69/wZwpmLyEDO4IzMxcDN3UzM1UTM1QDN5MjM5ADMwAjMwUzL3QzLxYzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![直線公理](/img/d/2c9/wZwpmLyQzNwMTO4UDM5IDN0UTMyITNykTO0EDMwAjMwUzL1AzLxAzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![直線公理](/img/8/3ed/wZwpmLxADN2QTO4MzNxADN0UTMyITNykTO0EDMwAjMwUzLzczL0YzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
解: 如圖1,設 邊上的高為 ,則
![直線公理](/img/a/6f5/wZwpmLycTNzADM3YzN2UzM1UTM1QDN5MjM5ADMwAjMwUzL2czL0QzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![直線公理](/img/f/d11/wZwpmLxcjM2QzN0QzM3UzM1UTM1QDN5MjM5ADMwAjMwUzL0MzL2YzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![直線公理](/img/d/2c9/wZwpmLyQzNwMTO4UDM5IDN0UTMyITNykTO0EDMwAjMwUzL1AzLxAzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![直線公理](/img/8/3ed/wZwpmLxADN2QTO4MzNxADN0UTMyITNykTO0EDMwAjMwUzLzczL0YzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![直線公理](/img/b/be3/wZwpmLxUTN2gTM5cTO4kzM0UTMyITNykTO0EDMwAjMwUzL3kzL1YzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![直線公理](/img/d/2c9/wZwpmLyQzNwMTO4UDM5IDN0UTMyITNykTO0EDMwAjMwUzL1AzLxAzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
邊上的高為 就是點 到 的距離。
![直線公理](/img/d/2c9/wZwpmLyQzNwMTO4UDM5IDN0UTMyITNykTO0EDMwAjMwUzL1AzLxAzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
邊所在直線的方程為
![直線公理](/img/2/6d9/wZwpmLzETO1YzMwYTN2UzM1UTM1QDN5MjM5ADMwAjMwUzL2UzL0IzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![直線公理](/img/7/2b0/wZwpmLzEzMxADN0UjN2UzM1UTM1QDN5MjM5ADMwAjMwUzL1YzL3QzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
即 。
![直線公理](/img/a/f07/wZwpmL0ATN5kTMzQzM3UzM1UTM1QDN5MjM5ADMwAjMwUzL0MzL2UzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![直線公理](/img/7/2b0/wZwpmLzEzMxADN0UjN2UzM1UTM1QDN5MjM5ADMwAjMwUzL1YzL3QzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
點 到 的距離
![直線公理](/img/b/342/wZwpmL3czN0YTN5gzN2UzM1UTM1QDN5MjM5ADMwAjMwUzL4czLyUzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![直線公理](/img/2/951/wZwpmLwYDM5UTN1gTN2UzM1UTM1QDN5MjM5ADMwAjMwUzL4UzLyEzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
因此, 。
![直線公理](/img/d/2c9/wZwpmLyQzNwMTO4UDM5IDN0UTMyITNykTO0EDMwAjMwUzL1AzLxAzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
實際上,在以上解法中求直線 的方程可以不用兩點式,可以運用 直線 公理“兩點確定一條直線”通過觀察簡潔求解:
![直線公理](/img/b/c03/wZwpmLzYjMwUTMxIDN3UzM1UTM1QDN5MjM5ADMwAjMwUzLyQzL2IzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![直線公理](/img/1/afa/wZwpmLwcTNyMjM0QDO2UzM1UTM1QDN5MjM5ADMwAjMwUzL0gzL1EzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![直線公理](/img/8/b0b/wZwpmLxIjMzUzN1UzMzEzM1UTM1QDN5MjM5ADMwAjMwUzL1MzL3AzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
由點 及 ,得點 均在直線
![直線公理](/img/9/40b/wZwpmLyMjMxcTOyYTO2UzM1UTM1QDN5MjM5ADMwAjMwUzL2kzLwczLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![直線公理](/img/9/40b/wZwpmLyMjMxcTOyYTO2UzM1UTM1QDN5MjM5ADMwAjMwUzL2kzLwczLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![直線公理](/img/9/40b/wZwpmLyMjMxcTOyYTO2UzM1UTM1QDN5MjM5ADMwAjMwUzL2kzLwczLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
上,由“兩點確定一條直線”知,直線AB就是直線 ,即直線AB的方程是 。
![直線公理](/img/4/9e7/wZwpmL2QDO1EDM1MjN2UzM1UTM1QDN5MjM5ADMwAjMwUzLzYzLxQzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![直線公理](/img/f/a22/wZwpmL0gDMzITN4UDN3UzM1UTM1QDN5MjM5ADMwAjMwUzL1QzLxMzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
【例2】 已知兩條相交直線 的交點是(5,3),求過兩點 的直線的方程。
![直線公理](/img/a/b5d/wZwpmLwYTO4ETOycjN2UzM1UTM1QDN5MjM5ADMwAjMwUzL3YzL4gzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![直線公理](/img/6/eab/wZwpmLwEDO0UDO0UjN2UzM1UTM1QDN5MjM5ADMwAjMwUzL1YzL2czLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![直線公理](/img/5/cc4/wZwpmL3YTOwMDOwUzN2UzM1UTM1QDN5MjM5ADMwAjMwUzL1czLwMzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![直線公理](/img/4/9e7/wZwpmL2QDO1EDM1MjN2UzM1UTM1QDN5MjM5ADMwAjMwUzLzYzLxQzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
解:可得 所以 (否則 ,兩條直線 重合),把兩個等式相減,得
![直線公理](/img/f/00a/wZwpmL1AjN3ITO1gDO2UzM1UTM1QDN5MjM5ADMwAjMwUzL4gzL2UzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
所以所求直線的斜率為
![直線公理](/img/f/5bb/wZwpmL2YDN4gzN4YzM3UzM1UTM1QDN5MjM5ADMwAjMwUzL2MzLyQzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
所求直線的方程為
![直線公理](/img/d/ff4/wZwpmLzQTN4gzN4EDN3UzM1UTM1QDN5MjM5ADMwAjMwUzLxQzLyMzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![直線公理](/img/c/2a4/wZwpmL1YTO0MjN3QzM3UzM1UTM1QDN5MjM5ADMwAjMwUzL0MzL3IzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![直線公理](/img/8/092/wZwpmL4gzMxkTM5MTO2UzM1UTM1QDN5MjM5ADMwAjMwUzLzkzLxgzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![直線公理](/img/4/9e7/wZwpmL2QDO1EDM1MjN2UzM1UTM1QDN5MjM5ADMwAjMwUzLzYzLxQzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![直線公理](/img/f/a22/wZwpmL0gDMzITN4UDN3UzM1UTM1QDN5MjM5ADMwAjMwUzL1QzLxMzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![直線公理](/img/2/adb/wZwpmL3UDMwMTOzMTO2UzM1UTM1QDN5MjM5ADMwAjMwUzLzkzL1EzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
簡解: 由“兩條相交直線 ”知.點“ ”是不同的兩點,還可得 。
![直線公理](/img/2/369/wZwpmLwADM3ITN1IzM3UzM1UTM1QDN5MjM5ADMwAjMwUzLyMzL4EzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![直線公理](/img/f/984/wZwpmL2YjNwQzM1ITO2UzM1UTM1QDN5MjM5ADMwAjMwUzLykzL1EzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![直線公理](/img/a/703/wZwpmLxETM2AzM0QDO2UzM1UTM1QDN5MjM5ADMwAjMwUzL0gzLzgzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![直線公理](/img/1/78d/wZwpmL3UTN2gjM4MDO2UzM1UTM1QDN5MjM5ADMwAjMwUzLzgzL2UzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![直線公理](/img/b/052/wZwpmL3ETMxcTM2MDO2UzM1UTM1QDN5MjM5ADMwAjMwUzLzgzL1QzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![直線公理](/img/a/703/wZwpmLxETM2AzM0QDO2UzM1UTM1QDN5MjM5ADMwAjMwUzL0gzLzgzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![直線公理](/img/5/c36/wZwpmL3MzM3gDM3czM2EzM1UTM1QDN5MjM5ADMwAjMwUzL3MzLzgzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![直線公理](/img/a/703/wZwpmLxETM2AzM0QDO2UzM1UTM1QDN5MjM5ADMwAjMwUzL0gzLzgzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![直線公理](/img/5/c36/wZwpmL3MzM3gDM3czM2EzM1UTM1QDN5MjM5ADMwAjMwUzL3MzLzgzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![直線公理](/img/a/703/wZwpmLxETM2AzM0QDO2UzM1UTM1QDN5MjM5ADMwAjMwUzL0gzLzgzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
由 知,點, 在直線 上;由 知,點 也在直線 上,所以由“兩點確定一條直線”知,直線 就是直線 ,即直線 的方程是 。
直線的相關公理
關於直線的基本性質,是用下面一組公理描述的 。
阿基米德公理
![直線公理](/img/f/19c/wZwpmLzIDN0MDM3kTN2UzM1UTM1QDN5MjM5ADMwAjMwUzL5UzLyczLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![直線公理](/img/a/e3f/wZwpmL1AzN1kDO3ATMwEDN0UTMyITNykTO0EDMwAjMwUzLwEzL4UzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
對於任意線段 ,如圖2,存在一個自然數 ,使得
![直線公理](/img/c/1f5/wZwpmLxgDO3IjM5QDM3UzM1UTM1QDN5MjM5ADMwAjMwUzL0AzL1AzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![圖2](/img/d/ed7/wZwpmL0UzN5YDNwQDN3UzM1UTM1QDN5MjM5ADMwAjMwUzL0QzLxEzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![直線公理](/img/5/f1c/wZwpmLwgzM0EDO5UDM3UzM1UTM1QDN5MjM5ADMwAjMwUzL1AzL1EzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
阿基米德公理是說,無論線段OP如何小,點M離點O無論多遠,用線段OP在直線上連續截取足夠的次數,將得到終點Q,點Q在點M的右邊,即 。
稠密性公理
![直線公理](/img/8/b0b/wZwpmLxIjMzUzN1UzMzEzM1UTM1QDN5MjM5ADMwAjMwUzL1MzL3AzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![直線公理](/img/b/be3/wZwpmLxUTN2gTM5cTO4kzM0UTMyITNykTO0EDMwAjMwUzL3kzL1YzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
對於直線上任意兩個不同的點 ,在直線上至少存在一點 介於點A和點B之間,如圖3。
![圖3](/img/1/856/wZwpmL0QjN2cTM0YDM3UzM1UTM1QDN5MjM5ADMwAjMwUzL2AzL1czLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
由公理很容易得到下面的推論。
推論 在直線上的點A與點B之間,存在著無限多個點。
連續性公理
![直線公理](/img/5/31f/wZwpmL0ATN5IDM4QDM3UzM1UTM1QDN5MjM5ADMwAjMwUzL0AzL3MzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
對於直線上的線段 的無窮序列,如果滿足:
![直線公理](/img/2/c8d/wZwpmL2MTN1UDMyYDM3UzM1UTM1QDN5MjM5ADMwAjMwUzL2AzL1EzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
(1) ;
![直線公理](/img/0/92c/wZwpmL3AzM1UzMyEDMyADN0UTMyITNykTO0EDMwAjMwUzLxAzL3UzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![直線公理](/img/a/e3f/wZwpmL1AzN1kDO3ATMwEDN0UTMyITNykTO0EDMwAjMwUzLwEzL4UzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![直線公理](/img/e/3e2/wZwpmLzETM2kTM2gTN2UzM1UTM1QDN5MjM5ADMwAjMwUzL4UzL2QzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
(2) 對於給定的任一小的線段 ,總存在一個數 使得 ,如圖4,
![直線公理](/img/9/cc2/wZwpmLzATN2MTMzIjM2EzM1UTM1QDN5MjM5ADMwAjMwUzLyIzL1IzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![直線公理](/img/f/bc3/wZwpmL1UTN1kTOwUzN2UzM1UTM1QDN5MjM5ADMwAjMwUzL1czLwYzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![直線公理](/img/a/e3f/wZwpmL1AzN1kDO3ATMwEDN0UTMyITNykTO0EDMwAjMwUzLwEzL4UzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
那么存在唯一的點 ,使得 (對一切 ),這個公理也叫做 退縮線段原理,圖5是它的幾何解釋 。
![圖4](/img/6/db4/wZwpmLyUDM1UDO0ADO2UzM1UTM1QDN5MjM5ADMwAjMwUzLwgzLyAzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![圖5](/img/f/520/wZwpmLxcDMwUDM5MzM3UzM1UTM1QDN5MjM5ADMwAjMwUzLzMzLxMzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)