a) $AD \cap AF$ mà $AD//BC;\ AF // BE \Rightarrow (ADF) //(BEC)$b) $NN' // AB // EF // (CDEF) \ \ (*)$Lại có $MM' // CD \Rightarrow \dfrac{AM'}{AD} = \dfrac{AM}{AC} \ (1)$$NN' //AB \Rightarrow \Rightarrow \dfrac{AN'}{AF} = \dfrac{BN}{BF} \ (2)$$AM = BN;\ AC = BF \Rightarrow \dfrac{AN'}{AF} = \dfrac{BN}{BF} \ (3)$Từ $(1,2,3) \Rightarrow \dfrac{AM'}{AD} = \dfrac{BN'}{BF} \Rightarrow M'N' // DF // (CDEF) \ \ (**)$Từ $(*;\ **)$ có đpcm
a) $AD \cap AF$ mà $AD//BC;\ AF // BE \Rightarrow (ADF) //(BEC)$b) $NN' // AB // EF // (CDEF) \ \ (*)$Lại có $MM' // CD \Rightarrow \dfrac{AM'}{AD} = \dfrac{AM}{AC} \ (1)$$NN' //AB \Rightarrow \Rightarrow \dfrac{AN'}{AF} = \dfrac{BN}{BF} \ (2)$$AM = BN;\ AC = BF \Rightarrow \dfrac{AN'}{AF} = \dfrac{BN}{BF} \ (3)$Từ $(1,2,3) \Rightarrow \dfrac{AM'}{AD} = \dfrac{BN}{BF} \Rightarrow M'N' // DF // (CDEF) \ \ (**)$Từ $(*;\ **)$ có đpcm
a) $AD \cap AF$ mà $AD//BC;\ AF // BE \Rightarrow (ADF) //(BEC)$b) $NN' // AB // EF // (CDEF) \ \ (*)$Lại có $MM' // CD \Rightarrow \dfrac{AM'}{AD} = \dfrac{AM}{AC} \ (1)$$NN' //AB \Rightarrow \Rightarrow \dfrac{AN'}{AF} = \dfrac{BN}{BF} \ (2)$$AM = BN;\ AC = BF \Rightarrow \dfrac{AN'}{AF} = \dfrac{BN}{BF} \ (3)$Từ $(1,2,3) \Rightarrow \dfrac{AM'}{AD} = \dfrac{BN
'}{BF} \Rightarrow M'N' // DF // (CDEF) \ \ (**)$Từ $(*;\ **)$ có đpcm