х+у=8 |×2
3х-2у= 4
2x+2y=16
3x-2y=4
2x+2у+3х-2у=16+4
5х=20
х=20/5
х=4
у=8-х
у= 8-4=4
Ответ: х=4 , у=4
12х-7у=2 |×2
9х+14у=7
24x-14y=4
9x+14y=7
24x-14y+9x+14y= 4+7
33x=11
x=11/33
x=1/3
y=(12x-2)/7
y= (12/3-2)/7 = 2/7
Ответ: х=1/3 , у=2/7.
3х+7у=10 |×2
2х+5у=-13 |×(-3)
6x+14y=20
-6x -15y= 39
6x+14y-6x-15y=20+39
-y=59
y=- 59
x=(10-7y)/3
x = ( 10 - 7 *(-59) ) /3 = (10+413)/3 = 141
Ответ: х=141 , у= -59
5х+4у=30 |×(-2)
10х-10у=-3
-10x -8y= -60
10x -10y= -3
-10x-8y +10x-10y=-60+(-3)
-18y=-63
y= (-63) /(-18)
y= 3.5
x=(30-4y)/5
x= (30- 4*3.5) /5 = 16/5 = 3.2
Ответ: x=3.2 , y=3.5
9x +7y = 5 |×(-3)
4x +3y= 2 11/63 |×7
-27x- 21y = -15
28x + 21y = 137/63 × 7/1
-27x -21y +28x +21y = -15 + 137/9
x= -15 + 15 2/9
x=2/9
y=(5-9x)/7
y= ( 5 - 9/1 × 2/9)/7 = (5-2)/7 =3/7
Ответ: х=2/9 , у= 3/7.