聯立方程 加減消元法

1.)
3x =5y-9......(1)
5x=3y+1......(2)
5*(1),15x-25y=-45..(3)
3*(2),15x-9y=3......(4)
(3)-(4),-16y=-48
y=3
代y=3入(2)
5x=3(3)+1
x=2


2.)
2x-9y=20..................(1)
5x+4y=-3.................(2)
5*(1),10x-45y=100...(3)
2*(2),10x+8y=-6.......(4)
(4)-(3),53y=-106
y=-2
代y=-2入(2)
2x+9(2)=20
x=1

3.)
5x+11y=43.............(1)
-2x+3y=5................(2)
2*(1),10x+22y=86...(3)
5*(2),-10x+15y=25..(4)
(3)+(4),37y=111
y=3
代y=3入(2)
-2x+3(3)=5
x=2

4.)
2y = x/5 +4...(1)
x+9y= -1.......(2)
5*(1),10y=x+20
-x+10y=20.....(3)
(2)+(3),19y=19
y=1
代y=1入(2)
x+9=-1
x=-10

5.)
x+y/4 =11...(1)
x/3 -y =- 5...(2)
4*(1),4x+y=44...(3)
(2)+(3),13x/3=39
x=9
代x=9入(3)
4*9+y=44
y=8

6.)
7x+5y =3x+2y=-1
7x+5y=1...(1)
3x+2y=1...(2)
2*(1),14x+10y=2....(3)
5*(2),15x+10y=5....(4)
(4)-(3),x=3
代x=3入(2)
3*3+2y=1
y=-4

7.)
11x-3y-31 =4x+5x =-11
11x-3y-31=-11...(1)
4x+5x=-11...(2)
由(2)
9x=-11
x=-11/9
代x=-11/9入(1)
11(-11/9)-3y=20
y=-301/27

8.)
4x+y/3 = x+9y/2 =7
4x+y/3=7......(1)
x+9y/2=7......(2)
4*(2),4x+18y=28...(3)
(3)-(1),53y/3=21
y=63/53
代y=63/53入(3)
4x+18(63/53)=28
x=175/106

9.)
3x-7y/4 = 3x-2y/3 =-6
3x-7y/4=-6....(1)
3x-2y/3=-6....(2)
(1)-(2),-13y/12=0
y=0
代y=0入(1)
3x=-6
x=-2

2015-01-12 19:05:04 補充：
第七至九題是否打錯題目????

2015-01-12 19:09:52 補充：
7.)
11x-3y-31 =4x+5y =-11
11x-3y=20...(1)
4x+5y=-11...(2)
4*(1),44x-12y=80...(3)
11*(2),44x+55y=-121...(4)
(4)-(3),67y=-201
y=-3
代y=-3入(2)
4x+5(-3)=-11
x=1

2015-01-13 18:16:24 補充：
8.)
4x+y/3 = x+9y/2 =7
4x+y/3=7......(1)
x+9y/2=7......(2)
3*(1),4x+y=21...(3)
2*(2),x+9y=14...(4)
9*(3),36x+9y=189...(5)
(5)-(4),35x=175
x=5
代x=5入(3)
4*5+y=21
y=1

2015-01-13 18:22:52 補充：
9.)
3x-7y/4 = 3x-2y/3 =-6
3x-7y/4=-6....(1)
3x-2y/3=-6....(2)
4*(1),3x-7y=-24...(3)
3*(2)3x-2y=-18...(4)
(4)-(3),5y=6
y=6/5
代y=6/5入(4)
3x-2*6/5=-18
x=-26/5

2015-01-13 18:26:00 補充：
下次記得在合理的位置加返D括號，否則回答者會唔解你的意思。

http://i.imgur.com/yHY4dEk.jpg

第7至9題是圖片的第22題至24題
第9題(24題)是答案係 x = - 26/5, y = 6/5