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Editorial
- Let $S_a = \frac{AE}{AB} + \frac{AF}{AC}$. Define $S_b,S_c$ similarly.
- Use pigeon hole principle.
- Let $M_a = \frac{AE}{AB} \times \frac{AF}{AC}$. Define $M_b,M_c$ similarly.
- Use AM-GM inequality to find relation between $S_x$ and $M_x$ for some $x$. If you don't know about AM-GM inequality, you can google about it.
- The area of $\triangle ABC=\frac{\sin{\angle EBD}}{2}\times AB \times BC$