Ответ:

1) 2x+y=8

x=y+1

2(y+1)+y=8

x=y+1

2y+2+y=8

x=y+1

3y=8-2

x=y+1

3y=6

x=y+1

y=2

x=2+1

y=2

x=3.

2) x-y=0

3x-3y=0

x-y=0

3x=3y

x-y=0

x=y

x=y=любое число.

3) 2x-2y=-2

x+y=-2

2x-2y=-2

x=-y-2

2(-y-2)-2y=-2

x=-y-2

-2y-4-2y=-2

x=-y-2

-4y=4-2

x=-y-2

-4y=2

x=-y-2

y=-1/2

x=1/2-2

y=-1/2

x=-1 целая 1/2

9) 2x+y=7

x-2y=1

y=7-2x

x-2y=1

y=7-2x

x-2(7-2x)=1

y=7-2x

x-14+4x=1

y=7-2x

5x=1+14

y=7-2x

5x=15

y=7-2x

x=3

y=7-2*3

x=3

y=7-6

x=3

y=1

x=3.