7x+2y= 21
-3x+8y= -9
find x and y?
2008-07-14 2:34 pm
回答 (10)
2008-07-14 2:42 pm
✔ 最佳答案
7x+2y= 21-3x+8y= -9
multiply top by -4
-28x + -8y = -84
-3x+8y= -9
add equations together
-31x + 0y = -93
solve for x = 3
plug x=3 into 7x+2y= 21
7(3) + 2y = 21
21 +2y = 21
2y = 0
y=0
so x=3, y=0
參考: I was a math major in college
2008-07-14 3:00 pm
7x + 2y = 21 (solve by using elimination)
-3x + 8y = -9
7x + 2y = 21
4(7x + 2y) = 4(21)
28x + 8y = 84
...28x + 8y = 84
– -3x + 8y = -9 (subtraction)
--------------------------
31x = 93
31x = 93
x = 93/31
x = 3
7x + 2y = 21
7(3) + 2y = 21
21 + 2y = 21
2y = 21 - 21
2y = 0
y = 0/2
y = 0
â´ x = 3 , y = 0
-3x + 8y = -9
7x + 2y = 21
4(7x + 2y) = 4(21)
28x + 8y = 84
...28x + 8y = 84
– -3x + 8y = -9 (subtraction)
--------------------------
31x = 93
31x = 93
x = 93/31
x = 3
7x + 2y = 21
7(3) + 2y = 21
21 + 2y = 21
2y = 21 - 21
2y = 0
y = 0/2
y = 0
â´ x = 3 , y = 0
2008-07-14 2:48 pm
x=3, y=0 triple checked
2008-07-14 2:46 pm
x=3
y=0
y=0
2008-07-14 2:41 pm
multiply Equation 1 by -4 to cancel out the "y" and get:
-28x-8y=-84 and -3x+8y=-9
Add em up, the y's cancel, leaving you with -31x=-93
so x=3
Plug into one of teh original equations and get y=0
Check your work
FINAL ANSWER: (3,0)
-28x-8y=-84 and -3x+8y=-9
Add em up, the y's cancel, leaving you with -31x=-93
so x=3
Plug into one of teh original equations and get y=0
Check your work
FINAL ANSWER: (3,0)
2008-07-14 2:41 pm
x=3,y=0
2008-07-14 2:40 pm
if x=0
2y=21
y=10.5
y=0
7x=21
x=3
-------------------
if x=0
8y=-9
y=8/9
y=0
-x3=-9
x= 3
2y=21
y=10.5
y=0
7x=21
x=3
-------------------
if x=0
8y=-9
y=8/9
y=0
-x3=-9
x= 3
2008-07-14 2:40 pm
7x + 2y = 21 (Eq1)
-3x + 8y = -9 (Eq2)
Multiply Eq1 by 4
28x + 8y = 84 (Eq 3)
Subtract Eq2 from Eq3
28x + 8y - (-3x + 8y) = 84 - - 9
28x + 8y + 3x - 8y = 93
31x = 93
x = 3
Put x=3 back into Eq1
7 * 3 + 2y = 21
21 + 2y = 21
2y = 0
y = 0
So, x = 3 and y = 0
-3x + 8y = -9 (Eq2)
Multiply Eq1 by 4
28x + 8y = 84 (Eq 3)
Subtract Eq2 from Eq3
28x + 8y - (-3x + 8y) = 84 - - 9
28x + 8y + 3x - 8y = 93
31x = 93
x = 3
Put x=3 back into Eq1
7 * 3 + 2y = 21
21 + 2y = 21
2y = 0
y = 0
So, x = 3 and y = 0
2008-07-14 2:39 pm
X=3 Y=0 I verified it twice on paper
2008-07-14 2:38 pm
x = 3, y = 0
7x+2y= 21
-3x+8y= -9
multiply 1st by -4
-28x - 8y = -84
add to 2nd
-28x - 8y = -84
-3x + 8y = -9
---------------
-31x = -93
x = 3
put back in either/both original equations
7x+2y= 21
-3x+8y= -9
21 + 2y = 21
-9 + 8y = -9
y has to equal 0
7x+2y= 21
-3x+8y= -9
multiply 1st by -4
-28x - 8y = -84
add to 2nd
-28x - 8y = -84
-3x + 8y = -9
---------------
-31x = -93
x = 3
put back in either/both original equations
7x+2y= 21
-3x+8y= -9
21 + 2y = 21
-9 + 8y = -9
y has to equal 0
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