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Math Help - Simultaneous Equation

  1. #1
    Newbie s2951's Avatar
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    Question Simultaneous Equation

    x-3y=14 -(1)
    4x+2y=0 -(2)

    What is x & y?
    thanks.
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  2. #2
    Senior Member Stroodle's Avatar
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    Which part are you having trouble with?

    From the first equation you can see that  x=14+3y
    So plug 14+3y into the second equation in the place of x then solve that for y. Once you have the value of y plug that back into either equation to get the value of x.
    Let us know if you have trouble with this.
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  3. #3
    Junior Member mrmohamed's Avatar
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    Quote Originally Posted by s2951 View Post
    x-3y=14 -(1)
    4x+2y=0 -(2)

    What is x & y?
    thanks.
    hi
    from (2) y = -2 x
    put y = -2x in (1)
    x + 6x = 14 7x = 14 x = 2 , y = -4
    Last edited by mrmohamed; December 1st 2009 at 02:10 AM.
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  4. #4
    Newbie s2951's Avatar
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    Quote Originally Posted by mrmohamed View Post
    hi
    from (2) y = -2 x
    put y = -2x in (1)
    x + 6x = 14 7x = 14 x = 2 , y = -4
    x-3y=14 (1)
    4x+2y=0 (2)

    (1)
    x=14+3y

    (1) into (2)
    4(14+3y)+2y=0
    56+12y+2y=0
    56+14y=0
    14y=-56
    y=-4

    (2)
    4x+2(-4)=0
    4x+-8=0
    4x=8
    x=2

    Got it, so i have to use Substitution Method. Can i use Elimination Method ?
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  5. #5
    Super Member Bacterius's Avatar
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    Wouldn't be useful here. Elimination method is only useful when the fact of adding or substracting together both equations gets rid of an unknown (like -2x in the first one and 2x in the second). Here you wouldn't get anything useful [I never ever use the elimination method, oddly enough I prefer using substitution in all problems ...] but if you are asked to do the Elimination method ...
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  6. #6
    Super Member
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    Smile

    \left\{\begin{matrix}<br />
x-3y=14\\ <br />
4x+2y=0<br />
\end{matrix}\right. \Leftrightarrow \left\{\begin{matrix}<br />
x-3y=14\\ <br />
2x+y=0<br />
\end{matrix}\right.
    Gauss Elimination,
    \Rightarrow \left\{\begin{matrix}<br />
x-3y=14\\ <br />
14y=-56<br />
\end{matrix}\right.
    Therefore,the solution is  [x=2,y=-4].
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