Giải các hệ phương trình sau

a) x - y= 2

3x +2y = 11

b) 4x - 3y = 16

2x + y = 18

Đáp án:

$\begin{array}{l}
a)\left\{ \begin{array}{l}
x - y = 2\\
3x + 2y = 11
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
2x - 2y = 4\\
3x + 2y = 11
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
5x = 15\\
y = x - 2
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
x = 3\\
y = 3 - 2 = 1
\end{array} \right.\\
Vậy\,\left( {x;y} \right) = \left( {3;1} \right)\\
b)\left\{ \begin{array}{l}
4x - 3y = 16\\
2x + y = 18
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
4x - 3y = 16\\
6x + 3y = 54
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
10x = 70\\
y = 18 - 2x
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
x = 7\\
y = 18 - 2.7 = 4
\end{array} \right.\\
Vậy\,\left( {x;y} \right) = \left( {7;4} \right)
\end{array}$