# ok, really dumb math question... need help.

Discussion in 'The Lounge' started by colbystephens, Jan 26, 2007.

1. ### colbystephens1 ton statusGMOTM Winner

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i can't remember how to solve more than 2 simultaneous equations. how would i solve something with the following form:??

3x+4y+2z=0
2x+18y-5z=3
5x-2y+8z=9

what are x, y, and z.

this isn't an actual problem i have, so i don't care about the solution to that one (if it actually exists), i just want to know what the process is for solving three simultaneous equations. i feel so dumb for not remembering this.

thanks!!

2. ### CDA 4553/4 ton status

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Please (Really) Excuse My Dear Aunt Sally.

1) Parenthesis

2) Root

3) Exponents

4) Multiply

5) Divide

7) Subtract

(In that order.)

3. ### ryan22re1/2 ton status

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Solve the first equation for z, then input z into the second, solve for x or y, then input (whatever variable you solved for) into the third equation. Repeat until you have solved for all three variables.

Pretty sure that's how it works. I was going to solve your hypothetical equations, but I lost interest very quickly. Has been a long time since I had to do math like that.

4. ### 76zimmerFlyin RatPremium MemberGMOTM Winner

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the last 3 letters in the alphabet.....

5. ### camiswelding1/2 ton status

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the process is

1) Isolate a variable
2) the resulting definition for the variable will be combination of two other variables.. but that doesnt really solve the problem

or

Set two variables to zero and solve for the third... then graph it on and x/y graph

math is made up of truths...you cant make up things where there are no solutions... you cant use the equations with each other because they have no relation.. what is x in one and y in another doesnt mean anything... they arent related unless you are given some information to relate them.. how does a person with two feet wear three shoes... it makes no sense...there has to be a way to find the value without the variables.. otherwise you will always have an undefined variable... you can set them to zero and put them in a coordinate plane.. but thats for each one and they dont relate

if Im wrong.. show me

or ask my wife.. shes a genius...literally

6. ### 1979jimmy3501/2 ton status

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solve it as a matrix turn it into a matrix and then solve it

7. ### colbystephens1 ton statusGMOTM Winner

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i'm aware of these definitions, but this isn't what i'm asking.

8. ### colbystephens1 ton statusGMOTM Winner

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you can't solve for z - you won't get a value, so you'll still end up with two variables in your equations

9. ### colbystephens1 ton statusGMOTM Winner

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you can use the equations w/ eachother. take this example:

3x + 4y = 0
5x + 2y = 1

multiply the whole second equation by two, and then subtract it:

2(5x + 2y = 0) this becomes 10x + 2y = 2

then subtract that from the first equation:

3x + 4y = 0
-(10x + 4y = 2)
_____________

-7x = 2

from here, solve for x, then plug it into the equation for y.

now, how do you do that w/ three equations??

10. ### 54inches1/2 ton status

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EXACTLY CORRECT.

11. ### justhorsinaround3/4 ton status

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12. ### camiswelding1/2 ton status

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The general answer for three equations is.. you cant... unless you set a variable... you can graph them as coordinates by,in each equation, setting two of the variables to zero and solving for the third and graphing that point..

if I understand the gaussian method he did just what I already described.. eventually setting two variables to zero... if so it doesnt explain the steps to a solution for the original problem as presented by colby.. I believe I am still correct in my assessment

but this is getting beyond my pay grade... Im still working on the radii/length/stretch of bending, using different size dies, .125 aluminum for a custom center console for my new dodge... now thats math that really means something to me

cam

13. ### colbystephens1 ton statusGMOTM Winner

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nope.

can some one explain the matrix way of doing it? i'm not familiar w/ that way at all.

14. ### ARAMP1Aviator Extraordinaire

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Close, but 2x0=0, not 2.

15. ### ARAMP1Aviator Extraordinaire

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This is correct. In your example, I solved 3x+4y+2z=0 for z=-3x/2-2y. I then plugged the value for z into the 5x+2y+8z=9 to get it in terms of x and y, which got me 19x/2+28y=3. I then solved this equation for y=-x/2-14/9. Now I have y in terms of x and can solve from there.

I ended up with x=419/81 y=671/162 and z=-2599/162. To check your answer, plug x, y, and z into any of the equations and it should work out correctly.

16. ### ryan22re1/2 ton status

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Thanks for saving me the math. I was just about to sit down and write it out.

17. ### colbystephens1 ton statusGMOTM Winner

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ok, i'm blind. thanks. oops.

18. ### ARAMP1Aviator Extraordinaire

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There is a way to do it where you can mulitiply each coefficient of the same variable by the least common denominator and subtract one equation from another to give one equation with only two variables and go from there, but I find the above way a bit easier to follow and harder to accidently mess up the algebra.

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