Đáp án:
\(B(3; \, 6).\)
Giải thích các bước giải:
\(\begin{array}{l}
G\left( {\frac{7}{3};\, - \frac{4}{3}} \right),\,\,M\left( {1;\,1} \right),\,\,N\left( {2; - 4} \right)\\
Goi\,\,A\left( {a;\,\,b} \right)\\
M\,\,la\,\,trung\,\,diem\,\,cua\,\,\,AB\\
\Rightarrow B\left( {2 - a;\,\,2 - b} \right)\\
N\,\,la\,\,\,trung\,\,diem\,\,cua\,\,AC\\
\Rightarrow C\left( {4 - a;\, - 8 - b} \right)\\
G\,\,\,la\,\,trong\,\,tam\,\,\,\Delta ABC\\
\Rightarrow \left\{ \begin{array}{l}
3.\frac{7}{3} = a + 2 - a + 4 - a\\
3.\left( { - \frac{4}{3}} \right) = b - b - 8 - b
\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}
a = - 1\\
b = - 4
\end{array} \right. \Rightarrow A\left( { - 1; - 4} \right).\\
\Rightarrow B\left( {3;\,\,6} \right).
\end{array}\)
