$\begin{equation*} \begin{cases} 4x + 5y = 1 | * (-5) &\\ 5x + 7y = 5 | * 4 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} -20x - 25y = -5 &\\ 20x + 28y = 20 & \end{cases} \end{equation*}$
−20x − 25y + 20x + 28y = −5 + 20
3y = 15
y = 15 : 3
y = 5
4x + 5y = 1
4x = 1 − 5y
4x = 1 − 5 * 5
4x = 1 − 25
4x = −24
x = −24 : 4
x = −6
Ответ: (−6;5)

$\begin{equation*} \begin{cases} 3x - 5y = 25 | * (-4) &\\ 4x - 3y = 37 | * 3 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} -12x + 20y = -100 &\\ 12x - 9y = 111 & \end{cases} \end{equation*}$
−12x + 20y + 12x − 9y = −100 + 111
11y = 11
y = 1
4x − 3y = 37
4x = 37 + 3y
4x = 37 + 3 * 1
4x = 37 + 3
4x = 40
x = 40 : 4
x = 10
Ответ: (10;1)

$\begin{equation*} \begin{cases} 7x + 5y = -5 | * (-5) &\\ 5x + 3y = 1 | * 7 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} -35x - 25y = 25 &\\ 35x + 21y = 7 & \end{cases} \end{equation*}$
−35x − 25y + 35x + 21y = 25 + 7
−4y = 32
y = 32 : (−4)
y = −8
5x + 3y = 1
5x = 1 − 3y
5x = 1 − 3 * (−8)
5x = 1 + 24
5x = 25
x = 25 : 5
x = 5
Ответ: (5;−8)

$\begin{equation*} \begin{cases} 4x - 3y = 12 | * 3 &\\ 3x - 4y = 30 | * (-4) & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} 12x - 9y = 36 &\\ -12x + 16y = -120 & \end{cases} \end{equation*}$
12x − 9y − 12x + 16y = 36 − 120
7y = −84
y = −84 : 7
y = −12
3x − 4y = 30
3x = 30 + 4y
3x = 30 + 4 * (−12)
3x = 30 − 48
3x = −18
x = −18 : 3
x = −6
Ответ: (−6;−12)