求救 : 三題代入法 10 點

1. 5r+2s=6------------(1)
3r=8s-1-------------(2)

(2): r=(8s-1)/ 3--------------(3)

put(3) into (1)

5((8s-1)/3) +2s=6
(40s-5)/3+2s=6
(40s-5)/3+6s/3=6
40s-5+6s=6X3
46s-5=18
46s=18+5
46s=23
s=0.5
r=1

2. 2p-5q=24---------(1)
4p+7q=31--------(2)

(2): p=(31-7q)/4---------------(3)

put (3) into (1)

2((31-7q)/4)-5q=24
(62-14q-20q)/4=24 <---------　通分母
62-34q=96
-34q=34
q= -1
p=19/2


1. 7x+3y=5 X5---------(1)
5x-4y=22 X7-------------(2)

(1): 35x+15y=25
(2):35x-28y=154

(1)-(2): 43y= -129
Y= -3
x= 2

[代入法]
1. 5r+2s=6 ---------1
3r=8s-1 ---------2

5r+2s=6
r=(8s-1)/3 ----------3

用3式代入1式,
5r+2s=6
(40s-5)/3+2s=6
40s-5+6s=18
46s =23
s =0.5

用s=0.5代入2式,
3r=8s-1
3r=4-1
3r=3
r=1

所以r=1
s=0.5


[代入法]
2. 2p-5q=24 --------1
4p+7q=31 --------2

2p-5q=24
p=(24+5q)/2 ---------3

用3式代入2式,
4p+7q=31
96+20q)/2+7q=31
96+20q+14q=62
34q= -34
q= -1

用q= -1代入1式,
2p-5q=24
2p-(-5)=24
2p+5=24
2p=19
p=9.5

所以 p=9.5
q= -1


[消元法]

1. 7x+3y=5 -------1
5x-4y=22 -------2

1: x4 28x+12y=20
+ 2: x3 15x-12y=66

(28x+12y)+(15x-12y)=20+66
43x = 86
x = 2

用x = 2 代入 2式,
5x-4y=22
10-4y=22
-4y=12
y= -3

所以 x = 2
y = -3


希望幫到你^^

1)
5r+2s=6----------(1)
3r=8s-1-----------(2)

from (2) r=(8s-1)/3 ---------(3)

sub (3) into (1)
[5(8s-1)]/3+2s=6
(40s-5)/3+2s=6
40s-5+6s=18
46s=23
s=1/2

sub s=1/2 into (2)
3r=8(1/2)-1
3r=4-1
3r=3
r=1


2)
2p-5q=24--------------(1)
4p+7q=31-------------(2)

from (1) p=(5q+24)/2 -----------(3)

sub (3) into (2)
4[5q+24)/2]-5q=31
2(5q+24)+7q=31
10q+48+7q=31
17q=-17
q= -1

sub q= -1 into (1)
2p-5(-1)=24
2p+5=24
2p=19
p=9.5


3)
7x+3y=5-----------(1)
5x-4y=22 -------------(2)

(1)x 4 28x+12y=20 ----------(3)
(2)x3 15x-12y=66 -------------(4)
(3)+(4) 43x=86
x=2

sub x=2 into (2)
5(2)-4y=22
10-4y=22
4y= -12
y= -3