\[ \frac{BG}{GC} = \frac{AM}{MB} \]
\[ \frac{AG}{GM} = \frac{AB}{MB} \]
\[ \frac{BG}{GC} = \frac{AG}{GM} \]
\[ \frac{BN}{NC} = \frac{BM}{MG} \]
\[ \frac{BN}{BC} = \frac{BM}{BG + GC} = \frac{BM}{AG} = \frac{BM}{2MG} \]
\[ \frac{BN}{BC} = \frac{BM}{\frac{1}{2}BG} = 2 \cdot \frac{BM}{BG} = 2 \cdot \frac{AM}{MB} = \frac{2AM}{AB} = \frac{1}{2} \]
Vậy ta đã chứng minh được \( \frac{BN}{BC} = \frac{1}{2} \).