Đáp án:
$\begin{array}{l}
a)\left\{ \begin{array}{l}
2x - y = - 9\\
3x + y = - 1
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
2x - y + 3x + y = - 9 + \left( { - 1} \right)\\
2x - y = - 9
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
5x = - 10\\
y = 2x + 9
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
x = - 2\\
y = 2.\left( { - 2} \right) + 9 = 5
\end{array} \right.\\
Vậy\,x = - 2;y = 5\\
b)\left\{ \begin{array}{l}
3x - y = 8\\
2x + 5y = 11
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
15x - 5y = 40\\
2x + 5y = 11
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
17x = 51\\
3x - y = 8
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
x = 3\\
y = 3x - 8 = 1
\end{array} \right.\\
Vậy\,x = 3;y = 1\\
c)\left\{ \begin{array}{l}
5x - 2y = 1\\
3x + 7y = 2
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
15x - 6y = 3\\
15x + 35y = 10
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
41y = 7\\
5x - 2y = 1
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
y = \dfrac{7}{{41}}\\
x = \dfrac{{1 + 2y}}{5} = \dfrac{{11}}{{41}}
\end{array} \right.\\
Vậy\,x = \dfrac{{11}}{{41}};y = \dfrac{7}{{41}}\\
d)\left\{ \begin{array}{l}
6x - 5y = - 2\\
5x + 3y = 5\dfrac{1}{2} = \dfrac{{11}}{2}
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
30x - 25y = - 10\\
30x + 18y = 33
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
43y = 43\\
6x - 5y = - 2
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
y = 1\\
x = \dfrac{{5y - 2}}{6} = \dfrac{1}{2}
\end{array} \right.\\
Vậy\,x = \dfrac{1}{2};y = 1
\end{array}$
