Trong mặt phẳng với hệ tọa độ Oxy, cho tam giác AB...

B

- Hướng dẫn giải

\(\begin{array}{l}\left\{ \begin{array}{l}A\left( {2;4} \right)\\C\left( {2;1} \right)\end{array} \right. \Rightarrow M\left( {2;\frac{5}{2}} \right)\\ \Rightarrow \overrightarrow {MB} = \left( {3; - \frac{5}{2}} \right) = \frac{1}{2}\left( {6; - 5} \right)\\ \Rightarrow MB:\left\{ \begin{array}{l}x = 5 + 6t\\y = - 5t\end{array} \right..\end{array}\)

Ta có: 

\(\begin{array}{l}N\left( {20;{y_N}} \right) \in BM\\ \Rightarrow \left\{ \begin{array}{l}20 = 5 + 6t\\{y_N} = - 5t\end{array} \right.\\ \Leftrightarrow \left\{ \begin{array}{l}t = \frac{5}{2}\\{y_N} = - \frac{{25}}{2}\end{array} \right.\end{array}\)