1.a) \frac{AM}{AB} = \frac{AN}{AC} => MN//BC (định li Talet)b) K thuộc MN, I thuộc BC => KM//IB => \frac{MK}{IB} = \frac{AM}{AB}tg tự: NK//IC => \frac{KN}{IC} = \frac{AN}{AC}có: \frac{AM}{AB} = \frac{AN}{AC} (theo gt) => \frac{MK}{IB} = \frac{KN}{IC}I là tđ BC=> IB=IC => MK=KN => K là tđ MN
1.a) $\frac{AM}{AB} = \frac{AN}{AC}$ => MN//BC (định li Talet)b) K thuộc MN, I thuộc BC => KM//IB => \frac{MK}{IB} = \frac{AM}{AB}tg tự: NK//IC => $\frac{KN}{IC} = \frac{AN}{AC}$có: $\frac{AM}{AB} = \frac{AN}{AC}$ (theo gt) => $\frac{MK}{IB} = \frac{KN}{IC}$I là tđ BC=> IB=IC => MK=KN => K là tđ MN