$\begin{equation*} \begin{cases} 2x + 11y = 15 &\\ 10x - 11y = 9 & \end{cases} \end{equation*}$
2x + 11y + 10x − 11y = 15 + 9
12x = 24
x = 2
2 * 2 + 11y = 15
11y = 15 − 4
11y = 11
y = 1
Ответ: x = 2, y = 1.

$\begin{equation*} \begin{cases} 8x - 17y = 4 &\\ -8x + 15y = 4 & \end{cases} \end{equation*}$
8x − 17y − 8x + 15y = 4 + 4
−2y = 8
y = −4
8x − 17 * (−4) = 4
8x + 68 = 4
8x = 4 − 68
8x = −64
x = −8
Ответ: x = −8, y = −4.

$\begin{equation*} \begin{cases} 4x - 7y = 30 &\\ 4x - 5y = 90 & \end{cases} \end{equation*}$
4x − 7y − (4x − 5y) = 30 − 90
4x − 7y − 4x + 5y = −60
−2y = −60
y = 30
4x − 7 * 30 = 30
4x − 210 = 30
4x = 30 + 210
4x = 240
x = 60
Ответ: x = 60, y = 30.

$\begin{equation*} \begin{cases} 13x - 8y = 28 &\\ 11x - 8y = 24 & \end{cases} \end{equation*}$
13x − 8y − (11x − 8y) = 28 − 24
13x − 8y − 11x + 8y = 4
2x = 4
x = 2
11 * 2 − 8y = 24
8y = 22 − 24
8y = −2
y = −0,25
Ответ: x = 2, y = −0,25.