c) -△AEF và △ABC có: \(\dfrac{AE}{AB}=\dfrac{AF}{AC}\)(△ABE∼△ACF), \(\widehat{BAC}\) chung.
\(\Rightarrow\)△AEF∼△ABC (c-g-c) \(\Rightarrow\dfrac{S_{AEF}}{S_{ABC}}=\left(\dfrac{EF}{BC}\right)^2\).
-△MFB và △MEC có: \(\widehat{FMB}=\widehat{EMC}\) , \(\widehat{MFB}=\widehat{MEC}=90^0\)
\(\Rightarrow\)△MFB∼△MEC (g-g) \(\Rightarrow\dfrac{MF}{ME}=\dfrac{MB}{MC}\).
-△MEF và △MCB có: \(\dfrac{MF}{MB}=\dfrac{ME}{MC}\left(\dfrac{MF}{ME}=\dfrac{MB}{MC}\right),\widehat{EMF}=\widehat{CMB}\)
\(\Rightarrow\)△MEF∼△MCB (c-g-c) \(\Rightarrow\dfrac{S_{MEF}}{S_{MCB}}=\left(\dfrac{EF}{BC}\right)^2\)
\(\dfrac{AK}{AD}.\dfrac{AE}{AC}=\dfrac{S_{AKE}}{S_{ADC}}=\dfrac{S_{AFK}}{D_{ADB}}=\dfrac{S_{AKE}+S_{AFK}}{S_{ADC}+S_{ADB}}=\dfrac{S_{AEF}}{S_{ABC}}=\left(\dfrac{EF}{BC}\right)^2\)
\(\dfrac{MK}{MD}.\dfrac{AE}{AC}=\dfrac{S_{MEK}}{S_{MDC}}=\dfrac{S_{MFK}}{S_{MDB}}=\dfrac{S_{MEK}+S_{MFK}}{S_{MDC}+S_{MDB}}=\dfrac{S_{MEF}}{S_{MCB}}=\left(\dfrac{EF}{BC}\right)^2\)
\(\Rightarrow\dfrac{AK}{AD}=\dfrac{MK}{MD}\Rightarrow AK.MD=MK.AD\)
