Solve linear equations with 2 unknowns

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ironape
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Solve linear equations with 2 unknowns

Postby ironape » Wed Oct 14, 2009 5:45 pm

Im just not seeing what im doing wrong to solve this. Can anyone take a look and let me know where I am going wrong with it?

4x + 1y = 16
-3(4x + 1y) = -3(16)
-12x + -3y = -48
2x + 3y = -24
-10x + 0y = -24
-10x = -24
-10/-10 = -24/-10
x = -2.4

4(-2.4) + 1y = 16
-9.6 + 1y = 16
(-9.6+9.6) + 1y = (16 + 9.6)
0 + 1y = 25.6
1y = 25.6

y=25.6
x=-2.4

4x + 1y = 16
4(-2.4) + 1(25.6) = 16
-9.6 + 25.6 = 16

2x + 3y = 24
2(-2.4) + 3(25.6) = 24
-4.8 + 76.8 = 72???

Works for one but not the other?

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stapel_eliz
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Postby stapel_eliz » Wed Oct 14, 2009 7:20 pm

ironape wrote:4x + 1y = 16
-3(4x + 1y) = -3(16)

What are the two original equations? Thank you! :wink:

ironape
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Re: Solve linear equations with 2 unknowns

Postby ironape » Mon Oct 19, 2009 1:56 pm

4x + 1y = 16

2X + 3Y = 24

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stapel_eliz
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Postby stapel_eliz » Mon Oct 19, 2009 2:28 pm

ironape wrote:4x + 1y = 16

2X + 3Y = 24

From what you'd posted earlier, I think you're trying to solve by substitution...? I will assume that you mean "X" and "Y" to be "x" and "y" respectively, since otherwise the system is not solveable. :oops:

Since the first equation can easily solved for "y=", that's probably the way to go.

. . . . .

Then plug this in, in place of "y", in the second equation:

. . . . .

Then solve the linear equation for the value of x, and back-solve then for the value of y. :wink:

ironape
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Re: Solve linear equations with 2 unknowns

Postby ironape » Thu Oct 22, 2009 2:01 pm

whats the answer so i can try and figure out what ive done wrong. The way ive worked the problem has been the same for 10 other problems and they all came out right. I just dont know what im missing with this one.

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stapel_eliz
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Postby stapel_eliz » Thu Oct 22, 2009 2:27 pm

To figure out where you may have gone wrong, we'll need to see your work. Please dtart from what was given to you earlier, and please provide a clear listing of your steps after that point.

Or, if you prefer to try solving by elimination, try multiplying the second row by , giving you:

. . . . .

. . . . .

Please show your work, starting with adding down, and then showing how you solved the resulting linear equation in , etc.

Thank you! :wink:

ironape
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Re: Solve linear equations with 2 unknowns

Postby ironape » Thu Oct 22, 2009 5:16 pm

Problem: 4x+y=16 2x+3y=24

im going to solve for x since i can easily remove y in the first equation by multiplying by -3

4x + 1y = 16
-3(4x + 1y) = -3(16)
-12x + -3y = -48

I get the above result which i will subtract the second equation from

-12x + -3y = -48
2x + 3y = 24
-10x + 0y = -24
-10x = -24
-10/-10x = -24/-10
1x = 12/5

At this point does x=12/5?

ironape
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Re: Solve linear equations with 2 unknowns

Postby ironape » Thu Oct 22, 2009 7:10 pm

I found where I messed up. I have to be careful when figuring if an answer is negative or positive. I made 2 mistakes which skewed the answers. Inputting the below answers will check out.

x=2.4
y=9.6

Thanks for taking the time to help me out


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