Đáp án:
$\begin{array}{l}
1)\left\{ \begin{array}{l}
3x - y = - 1\\
- 3x + 2y = 5
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
3x - y - 3x + 2y = - 1 + 5\\
3x - y = - 1
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
y = 4\\
x = \frac{{y - 1}}{3}
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
y = 4\\
x = 1
\end{array} \right.\\
Vậy\,\left( {x;y} \right) = \left( {1;4} \right)\\
2)3{x^2} - 5x = 0\\
\Rightarrow x\left( {3x - 5} \right) = 0\\
\Rightarrow \left[ \begin{array}{l}
x = 0\\
3x - 5 = 0
\end{array} \right.\\
\Rightarrow \left[ \begin{array}{l}
x = 0\\
x = \frac{5}{3}
\end{array} \right.
\end{array}$
3)
Để pt có nghiệm dương thì:
$\begin{array}{l}
\left\{ \begin{array}{l}
\Delta \ge 0\\
\frac{{ - b}}{a} > 0\\
\frac{c}{a} > 0
\end{array} \right. \Rightarrow \left\{ \begin{array}{l}
{\left( { - 5} \right)^2} - 4.3.\left( { - 7m} \right) \ge 0\\
\frac{5}{3} > 0\left( {ld} \right)\\
\frac{{ - 7m}}{3} > 0
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
25 + 84m \ge 0\\
- 7m > 0
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
m \ge - \frac{{25}}{{84}}\\
m < 0
\end{array} \right.\\
Vậy\, - \frac{{25}}{{84}} \le m < 0
\end{array}$
