Đáp án:

 a. \(\left\{ \begin{array}{l}
x = \frac{9}{{11}}\\
y = \frac{7}{{11}}
\end{array} \right.\)

b. x=2,y=0,5

Giải thích các bước giải:

\(\begin{array}{l}
a.\left\{ \begin{array}{l}
3x + 4y = 5\\
4x - 2y = 2
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
3x + 4y = 5\\
2x - y = 1
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
3x + 4y = 5\\
y = 2x - 1
\end{array} \right.\\
 \leftrightarrow \left\{ \begin{array}{l}
3x + 4(2x - 1) = 5\\
y = 2x - 1
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
3x + 8x - 4 = 5\\
y = 2x - 1
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
11x = 9\\
y = 2x - 1
\end{array} \right.\\
 \leftrightarrow \left\{ \begin{array}{l}
x = \frac{9}{{11}}\\
y = 2.\frac{9}{{11}} - 1 = \frac{7}{{11}}
\end{array} \right.\\
b.\left\{ \begin{array}{l}
0,3x - 0,2y = 0,5\\
0,5x + 0,4y = 1,2
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
1,5x - y = 2,5\\
0,5x + 0,4y = 1,2
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
y = 1,5x - 2,5\\
0,5x + 0,4y = 1,2
\end{array} \right.\\
 \leftrightarrow \left\{ \begin{array}{l}
y = 1,5x - 2,5\\
0,5x + 0,4(1,5x - 2,5) = 1,2
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
y = 1,5x - 2,5\\
1,1x = 2,2
\end{array} \right. \leftrightarrow \left\{ \begin{array}{l}
x = 2\\
y = 0,5
\end{array} \right.\\

\end{array}\)