Pt hoành độ giao điểm:
\(2x+m=\frac{x+3}{x+1}\Leftrightarrow2x^2+\left(m+1\right)x+m-3=0\)
\(\Delta=\left(m+1\right)^2-8\left(m-3\right)=\left(m-3\right)^2+16>0\)
\(\left\{{}\begin{matrix}x_1+x_2=-\frac{m+1}{2}\\x_1x_2=\frac{m-3}{2}\end{matrix}\right.\)
Ta có: \(MN^2=\left(x_1-x_2\right)^2+\left(y_1-y_2\right)^2\)
\(=5\left(x_1-x_2\right)^2=5\left(x_1+x_2\right)^2-20x_1x_2\)
\(=5\left(-\frac{m+1}{2}\right)^2-20\left(\frac{m-3}{2}\right)=\frac{5}{4}\left(m-3\right)^2+20\ge20\)
Dấu "=" xảy ra khi \(m=3\)
