初中數學問題

1 y = 3x - 15

7x + 2(3x - 15) - 7 = 15

7x + 6x - 30 - 7 = 15

13x - 37 = 15

13x = 52

x = 4

3x - y = 15

12 - y = 15

y = -3

1. 7x+2y-7=3x-y=15
=>7x+2y-7=3x-y and 3x-y=15
=>4x+3y=7 -(1) and 3x-y=15 -(2)
from (2),
y=3x-15
put into (1)
4x+3(3x-15)=7
4x+9x-45=7
13x=52
x=4//

put x=4 into (2)
y=3x-15=3(4)-15=-3//
so x=4,y=-3//

2. 5x-3y-11=4x-7y-18=0
=>5x-3y-11=0 and 4x-7y-18=0
=>5x-3y=11(1) and 4x-7y=18(2)
4(1)-5(2):
20x-12y-20x+35y=44-90
23y=-46
y=-2//
5x-3y=11
5x+6=11
x=9/5//

2(x+y)-3x=4--(1)
3(x+y)+5y=12--(2)
(1) become 2x+2y-3x=4 --> 2y-x=4 -->x=2y-4(3)
(2) becomes 3x+3y+5y=12 -->3x+8y=12 (4)
put (3) into (4) :
3(2y-4)+8y=12
6y-12+8y=12
14y=24
y=12/7//

x=2(12/7)-4=-4/7//

姐係分為2條equations?
1)
7x+2y-7=3x-y=15
1-step: 7x+2y-7=15------(1)
3x-y=15------(2)
2-step: 7x+2y=22--------(1)'
3x-y=15--------(2)'
3-step: (2)'X2: 6x-2y=30---(3)
4-step: (1)'-(3): x=-8
5-step: Sub x=-8 into (2)
6-step: 3x-y=15
3(-8)-y=15
-24-y=15
-y=15+24
y=-49
係咪咁!?