Question

Giúp em bài nầy với ạ. Em cảm ơn ạ

Answer 1

Hệ có nghiệm duy nhất khi \(m^2\ne1\Rightarrow m\ne\pm1\)

Khi đó: \(\left\{{}\begin{matrix}x+my=m+1\\m^2x+my=3m^2-m\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x+my=m+1\\\left(m^2-1\right)x=3m^2-2m-1\end{matrix}\right.\)

\(\Rightarrow\left\{{}\begin{matrix}x=\dfrac{3m+1}{m+1}\\y=\dfrac{m-1}{m+1}\end{matrix}\right.\)

Đặt \(P=xy=\dfrac{\left(3m+1\right)\left(m-1\right)}{\left(m+1\right)^2}=\dfrac{3m^2-2m-1}{\left(m+1\right)^2}=\dfrac{-\left(m+1\right)^2+4m^2}{\left(m+1\right)^2}\)

\(=-1+\left(\dfrac{2m}{m+1}\right)^2\ge-1\)

\(P_{min}=-1\) khi \(m=0\)