Đáp án:
$\begin{array}{l}
a)\left\{ \begin{array}{l}
4x + 5y = 3\\
x - 3y = 5
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
4x + 5y = 3\\
x = 3y + 5
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
4.\left( {3y + 5} \right) + 5y = 3\\
x = 3y + 5
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
12y + 20 + 5y = 3\\
x = 3y + 5
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
17y = - 17\\
x = 3y + 5
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
y = - 1\\
x = - 3 + 5 = 2
\end{array} \right.\\
Vậy\,x = 2;y = - 1\\
b)\left\{ \begin{array}{l}
7x - 2y = 1\\
3x + y = 6
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
7x - 2y = 1\\
6x + 2y = 12
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
13x = 13\\
y = 6 - 3x
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
x = 1\\
y = 6 - 3.1 = 3
\end{array} \right.\\
Vậy\,x = 1;y = 3\\
c)\left\{ \begin{array}{l}
1,7x - 2y = 3,8\\
2,1x + 2,5y = 5,5
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
4,25x - 5y = 9,5\\
4,2.x + 5y = 11
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
8,45x = 20,5\\
2y = 1,7x - 3,8
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
x = \dfrac{{410}}{{169}}\\
y = \dfrac{{137}}{{845}}
\end{array} \right.\\
Vậy\,x = \dfrac{{410}}{{169}};y = \dfrac{{137}}{{845}}
\end{array}$
