定理定義
![梅涅勞斯定理](/img/9/cb5/wZwpmL4QjNyMjM2MzM2EzM1UTM1QDN5MjM5ADMwAjMwUzLzMzLxIzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![梅涅勞斯定理](/img/a/1f1/wZwpmL3gzMzgjN0QzM2EzM1UTM1QDN5MjM5ADMwAjMwUzL0MzLzczLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![梅涅勞斯定理](/img/8/7ea/wZwpmL4ADO0QzN1czM2EzM1UTM1QDN5MjM5ADMwAjMwUzL3MzL4QzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
當一條直線交三邊所在的直線分別於點時,則有
![梅涅勞斯定理](/img/9/a26/wZwpmLxAzNyETO1UjM2EzM1UTM1QDN5MjM5ADMwAjMwUzL1IzL1YzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
定理證明
證明一
過點A作AG∥DF交BC的延長線於點G.則
![梅涅勞斯定理](/img/5/1fe/wZwpmLzMDNyQDO3MjM2EzM1UTM1QDN5MjM5ADMwAjMwUzLzIzLxczLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
證明二
![梅涅勞斯定理](/img/4/ab4/wZwpmLzIDNxQDN3IjM2EzM1UTM1QDN5MjM5ADMwAjMwUzLyIzLxEzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![梅涅勞斯定理](/img/e/4c4/wZwpmLwMzNwgjMxEjM2EzM1UTM1QDN5MjM5ADMwAjMwUzLxIzL0YzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
過點C作CP∥DF交AB於P,則
![梅涅勞斯定理](/img/b/913/wZwpmLwczN2kDO1gTM2EzM1UTM1QDN5MjM5ADMwAjMwUzL4EzLxgzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
兩式相乘得
證明三
![梅涅勞斯定理](/img/6/b93/wZwpmLzczMwUzN4gTM2EzM1UTM1QDN5MjM5ADMwAjMwUzL4EzL0AzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
連結CF、AD,根據“兩個三角形等高時面積之比等於底邊之比”的性質有。
AF:FB =S:S…………(1)
BD:DC=S:S…………(2),
CE:EA=S:S=S:S=(S+S
):(S+S)
=S:S………… (3)
(1)×(2)×(3)得
![梅涅勞斯定理](/img/0/aaa/wZwpmLxADO1MTO3MzM2EzM1UTM1QDN5MjM5ADMwAjMwUzLzMzLzEzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![梅涅勞斯定理](/img/3/e90/wZwpmLzMDOygDNxIjM2EzM1UTM1QDN5MjM5ADMwAjMwUzLyIzL2MzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![梅涅勞斯定理](/img/3/71f/wZwpmL0YDO2MzN3AjM2EzM1UTM1QDN5MjM5ADMwAjMwUzLwIzL3IzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![梅涅勞斯定理](/img/7/8f6/wZwpmL0EjNwkTNyQTM2EzM1UTM1QDN5MjM5ADMwAjMwUzL0EzL4AzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![梅涅勞斯定理](/img/3/d94/wZwpmL0YTNxgDOyQzM2EzM1UTM1QDN5MjM5ADMwAjMwUzL0MzL3gzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![梅涅勞斯定理](/img/0/e78/wZwpmL1cDOxATN2UzM2EzM1UTM1QDN5MjM5ADMwAjMwUzL1MzLwgzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![梅涅勞斯定理](/img/b/0a5/wZwpmLyMDMyUDNzADN2EzM1UTM1QDN5MjM5ADMwAjMwUzLwQzL1UzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
× × = × ×
證明四
![梅涅勞斯定理](/img/0/bfa/wZwpmL0gDMwIDM3gzM2EzM1UTM1QDN5MjM5ADMwAjMwUzL4MzLwgzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
過三頂點作直線DEF的垂線AA‘,BB',CC',如圖:
充分性證明:
△ABC中,BC,CA,AB上的分點分別為D,E,F。
![梅涅勞斯定理](/img/b/548/wZwpmL3ITM5QjM0EDN2EzM1UTM1QDN5MjM5ADMwAjMwUzLxQzLxQzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
連線DF交CA於E',則由充分性可得,
![梅涅勞斯定理](/img/9/a26/wZwpmLxAzNyETO1UjM2EzM1UTM1QDN5MjM5ADMwAjMwUzL1IzL1YzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
又∵
![梅涅勞斯定理](/img/d/9cd/wZwpmLxYzM3cTM1MzM2EzM1UTM1QDN5MjM5ADMwAjMwUzLzMzL2QzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
∴有CE/EA=CE'/E'A,兩點重合。所以 共線
![梅涅勞斯定理](/img/7/9bc/wZwpmLzUzM5cjM5MDN2EzM1UTM1QDN5MjM5ADMwAjMwUzLzQzL2IzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![梅涅勞斯定理](/img/e/13b/wZwpmLwADN1QDMwIjM2EzM1UTM1QDN5MjM5ADMwAjMwUzLyIzLyEzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![梅涅勞斯定理](/img/d/d50/wZwpmLwADO2cDM1cTM2EzM1UTM1QDN5MjM5ADMwAjMwUzL3EzL3AzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
推論 在△ABC的三邊BC、CA、AB或其延長線上分別取L、M、N三點,又分比是λ= 、μ= 、ν= 。於是L、M、N三點共線的充要條件是λμν=-1。(注意與塞瓦定理相區分,那裡是λμν=1)
![梅涅勞斯定理](/img/8/c9b/wZwpmL1YDO4IzMzcTM2EzM1UTM1QDN5MjM5ADMwAjMwUzL3EzL4IzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
此外,用該定理可使其容易理解和記憶:
第一角元形式的梅涅勞斯定理如圖:若E,F,D三點共線,則
![梅涅勞斯定理](/img/3/792/wZwpmLygDOwQDO3EDN2EzM1UTM1QDN5MjM5ADMwAjMwUzLxQzL2AzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
即圖中的藍角正弦值之積等於紅角正弦值之積。
該形式的梅涅勞斯定理也很實用。
證明:可用面積法推出:第一角元形式的梅氏定理與頂分頂形式的梅氏定理等價。
第二角元形式的梅涅勞斯定理
![梅涅勞斯定理](/img/c/fab/wZwpmLyEDN1UzNygTM2EzM1UTM1QDN5MjM5ADMwAjMwUzL4EzL3QzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
在平面上任取一點O,且EDF共線,則 (O不與點A、B、C重合)
![梅涅勞斯定理](/img/e/4fc/wZwpmL4MzN3kTN4YTM2EzM1UTM1QDN5MjM5ADMwAjMwUzL2EzLzYzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
定理意義
使用梅涅勞斯定理可以進行直線形中線段長度比例的計算,其逆定理還可以用來解決三點共線、三線共點等問題的判定方法,是平面幾何學以及射影幾何學中的一項基本定理,具有重要的作用。梅涅勞斯定理的對偶定理是塞瓦定理。
它的逆定理也成立:若有三點F、D、E分別在的邊AB、BC、CA或其延長線上,且滿足AF/FB×BD/DC×CE/EA=1,則F、D、E三點共線。利用這個逆定理,可以判斷三點共線。