$4c. theo simson=>FK vuong gocDC=>FIDK: la hinh chu nhat.\frac{BD}{DK}+\frac{CD}{FK}=\frac{BD}{DK}+\frac{CD}{DI}=\frac{DI-IB}{DK}+\frac{DK+KC}{DI}=\frac{DI}{DK}+\frac{DK}{DI}+\frac{KC}{DI}-\frac{BI}{IF}.Laico:\Delta BIF\sim \Delta CKF=>\frac{BI}{IF}=\frac{CK}{KF}=\frac{KC}{DI}=>\frac{BD}{DK}+\frac{CD}{FK}\geq 2.Dedangco \frac{BC}{FH}\geq 2=>P\geq 4.$
Rất hay. Cảm ơn bạn.
- NguyenDangHuyYTNA yêu thích