聯立二元一次方程

Q1)
x-y=-2-----(1)
y=x+2

2x+y=5----(2)

sub (1) into (2)
2x+x+2=5
3x=3
x=1
y=3

Q2)
2x-3y=7----(1)
6x=21+9y

3x-7y=23--(2)
6x=46+14y

21+9y =46+14y
5y=-25
y=-5
x=-4

Q3)
0.5x-0.2y=1.3----(1)
x=2.6+0.4y

x-y=2--------------(2)
x=2+y

2.6+0.4y =2+y
0.6=0.6y
y=0.1
x=3

1 . from (1) y=x+2, subs. y into (2)
2x +x+2=5
x=1,
from (2)
2x1+y=5
y=3

2 3x(1) - 2x(2)
6x-9y =21
6x-14y=46
5y=-25, y=-5
2x-3(-5)=7
x=-4

3 2 x (1) - (2)
x-0.4y=2.6
x-y=2
0.6y=0.6
y=1
then x=3