\(d:x+y+2=0\Rightarrow\overrightarrow{n_d}=\left(1;1\right)\)
\(A\in AD\Rightarrow A\left(a;2a+1\right)\) ;\(B\in BM\Rightarrow B\left(b;-3\right)\)
Gọi H là trung điểm AB \(\Rightarrow H\left(\frac{a+b}{2};a-1\right)\)
Do H thuộc trung trực AB:
\(\Rightarrow\frac{a+b}{2}+a-1+2=0\Leftrightarrow3a+b+2=0\)
\(\overrightarrow{AB}=\left(b-a;-4-2a\right)\) mà AB vuông góc d
\(\Rightarrow\overrightarrow{AB}.\overrightarrow{n_d}=0\Leftrightarrow b-a-4-2a=0\Leftrightarrow3a-b+4=0\)
\(\left\{{}\begin{matrix}3a+b+2=0\\3a-b+4=0\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}a=-1\\b=1\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}A\left(-1;-1\right)\\B\left(1;-3\right)\end{matrix}\right.\)
Phương trình BC: \(1\left(x-1\right)+2\left(y+3\right)=0\Leftrightarrow x+2y+5=0\)
\(\Rightarrow C\left(2c-5;-c\right)\Rightarrow M\left(c-3;\frac{-c-1}{2}\right)\)
Mà \(M\in BM\Rightarrow\frac{-c-1}{2}+3=0\Leftrightarrow-c+5=0\Rightarrow c=5\Rightarrow C\left(5;-5\right)\)