有數學唔識做呀!急求人幫我

用代入法來去求X同Y
5X-Y=3_______(1)
3X+Y=21_______(2)
from(2),Y=21-3X_________(3)
Sub(3) into (1),5X-(21-3X)=3
5X-21+3X=3
8X=3+21
X=3
Put X=3 into (2),3(3)+Y=21
9+Y=21
Y=12
.'.The solution is X=3,Y=12

用消元法來去求X同Y
1.4X-Y=5________(1)
3X-Y=-1__________(2)
(1)-(2),(4X-Y)-(3X-Y)=5-(-1)
X=6
Put X=6 into (2),3(6)-Y=-1
18-Y=-1
-Y=-19
Y=19
.'.The solution is X=6,Y=19

2.X-4Y=2___________(1)
2X-Y=11_____________(2)
(1)*2,2(X-4Y)=2(2)
2X-8Y=4___________(3)
(2)-(3),(2X-Y)-(2X-8Y)=11-4
7Y=7
Y=1
Put Y=1 into (2),2X-1=11
2X=12
X=6
.'.The solution is X=6,Y=1

3.7X+2Y=3__________(1)
Y-X=3____________(2)
(2)*2,2(Y-X)=3*2
2Y-2X=6_________(3)
(1)-(3),(7X+2Y)-(2Y-2X)=3-6
9X=-3
X=-1/3
Put X=-1/3 into (2),Y-(-1/3)=3
Y+1/3=3
Y=3 1/3(3又3分之1...唔知點打/o\)
.'.The solution is X=-1/3,Y=3 1/3

用代入法來去求X同Y
1.
5X-Y=3_______(1)
3X+Y=21_______(2)

5x-y = 3
y = 5x-3 ------------(3)

將(3)代入(1)

3x+y = 21
3x+(5x-3) = 21
3x+5x-3 = 21
8x = 24
x = 3
================


用消元法來去求X同Y
2(A).
4X-Y=5________(1)
3X-Y=-1__________(2)

4x - Y = 5
-) 3x -Y = -1
--------------------------------
x = 6

將 x=6代入(1)

4(6)-Y=5
24-5 =Y
Y= 19

========================================

2.(B).
X-4Y=2___________(1)
2X-Y=11_____________(2)

(1) x2 :
X-4Y =2
2X -8Y =4 ----------------(3)

(3)-(2)

2X-8Y = 4
-) 2X-Y =11
-------------------------
-7Y = -7
Y= 1

PUT Y=1 INTO (1)
X-4(1)=2
X=6
=============================

2(C).
7X+2Y=3__________(1)
Y-X=3____________(2)

(2)x2 :
Y-X=3
2Y -2X =6-------------(3)

(1)-(3)

7X+2Y =3
-) -2X+2Y =6
--------------------------------
9X =-3
X = - 1/3

Put X= -1/3 INTO (1)
7(-1/3) +2Y =3
-7/3 +2Y =3
2Y = 16/3
Y= 8/3
===========

1)From (1), Y=5X-3
Substitute Y=5X-3 into (2)
3X+5X-3=21
X=3
So Y=5(3)-3=12
2(A)(1)-(2):
(4X-Y)-(3X-Y)=5-(-1)
X=6
Substiture X=6,we have
4(6)-Y=5
Y=19
2(B) (1)x2: 2X-8Y=4--(3)
(2)-(3): 7Y=7
Y=1
So X=(11+1)/2=6
2(C) (2)x7: 7Y-7X=21---(3)
(1)+(3):
7X+2Y+7Y-7X=3+21
9Y=24
Y=8/3
So x=8/3-3=-1/3