\Delta ABE \infty \Delta ACF (g.g) \rightarrow \frac{BE}{FC}=\frac{AE}{AF}(1) mà BE//FC \rightarrow \frac{KB}{KF}= \frac{BE}{FC} từ (1)\rightarrow \frac{AE}{AF}= \frac{KB}{KF}ap dung dinh li ta-let dao \Delta AFK \rightarrow AK//BE\rightarrow AK\perp AD[/tex]