孟氏定理或梅涅劳斯定理

$$\large \frac{AN}{BN} \cdot \frac{BL}{CL} \cdot \frac{CM}{AM} = 1$$

證明

透過正弦定理，有 $$\frac{AN}{AM} = \frac{\sin\beta}{\sin\alpha}$$ 同理可得， $$\frac{BL}{BN} = \frac{\sin\alpha}{\sin\gamma} , \frac{CM}{CL} = \frac{\sin\gamma}{\sin\beta}$$ 相乘即可得 $$\frac{AN}{AL} \cdot \frac{BL}{BN} \cdot \frac{CM}{CL} = \frac{\sin\beta}{\sin\alpha} \cdot \frac{\sin\alpha}{\sin\gamma} \cdot \frac{\sin\gamma}{\sin\beta} = 1$$

參考資料: Wikipedia