Đáp án:
\[m > 2\sqrt 2 - 2\]
Giải thích các bước giải:
\[\begin{array}{l}
Cau\,\,2:\\
y = \frac{{2{x^2} + mx + 2 - m}}{{x + m - 1}}\,\,\,DB\,\,\,tren\,\,\,\left( {1; + \infty } \right)\\
DK:\,\,\,x \ne 1 - m\\
\Rightarrow y' = \frac{{\left( {4x + m} \right)\left( {x + m - 1} \right) - \left( {2{x^2} + mx + 2 - m} \right)}}{{{{\left( {x + m - 1} \right)}^2}}}\\
= \frac{{4{x^2} + 4\left( {m - 1} \right)x + mx + {m^2} - m - 2{x^2} - mx - 2 + m}}{{{{\left( {x + m - 1} \right)}^2}}}\\
= \frac{{2{x^2} + 4\left( {m - 1} \right)x + {m^2} - 2}}{{{{\left( {x + m - 1} \right)}^2}}}.\\
\Rightarrow y' = 0\\
\Leftrightarrow 2{x^2} + 4\left( {m - 1} \right)x + {m^2} - 2 = 0\\
\Delta ' = 4{\left( {m - 1} \right)^2} - 2{m^2} + 4 = 4{m^2} - 8m + 4 - 2{m^2} + 4\\
= 2{m^2} - 8m + 8 = 2\left( {{m^2} - 4m + 4} \right) = 2{\left( {m - 2} \right)^2}.\\
\Rightarrow \Delta ' > 0 \Leftrightarrow m \ne 2\\
Bang\,\,xet\,\,dau:\\
\,\,\,\,\, + \,\,\,\,\,\,\,\,{x_1}\,\,\,\,\, - \,\,\,\,\,\,\,\,\,{x_2}\,\,\,\,\,\, + \\
\Rightarrow hs\,\,\,DB\,\,\,tren\,\,\,\left( {1;\,\, + \infty } \right)\\
\Leftrightarrow \left\{ \begin{array}{l}
{x_1} < {x_2} < 1\\
1 - m \notin \left( {1; + \infty } \right)
\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}
{x_1} + {x_2} < 2\\
\left( {{x_1} - 1} \right)\left( {{x_2} - 1} \right) > 0\\
1 - m \le 1
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
- 2\left( {m - 1} \right) < 2\\
{x_1}{x_2} - \left( {{x_1} + {x_2}} \right) + 1 > 0\\
m \ge 0
\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}
m - 1 > - 1\\
\frac{{{m^2} - 2}}{2} + 2\left( {m - 1} \right) + 1 > 0\\
m \ge 0
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
m > 0\\
m \ge 0\\
{m^2} - 2 + 4m - 4 + 2 > 0
\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}
m > 0\\
{m^2} + 4m - 4 > 0
\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}
m > 0\\
\left[ \begin{array}{l}
m > - 2 + 2\sqrt 2 \\
m < - 2 - 2\sqrt 2
\end{array} \right.
\end{array} \right. \Leftrightarrow m > 2\sqrt 2 - 2.
\end{array}\]