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System of linear equations

  1. Apr 18, 2005 #1
    I'm hoping someone can come along and give me a tap on the head for being so silly, but i've been trying this problem *all day* and i just cant seem to get a correct answer, even when checked with other people, we're all stumped! What we have:

    [itex]3x - 2y + 5z = 0[/itex]
    [itex]x + y + 5z = 5[/itex]
    [itex]x - 2y - z = -4[/itex]

    I've then put this into an augmented matrix, and converted to reduce row echelon form which gives me these equations:

    [itex]x + z = 2[/itex]
    [itex]y + z = 3[/itex]

    So I try a general solution set as (t, 1+t, 2-t) however it doesn't seem to work with the equations at the beginning. so i looked at what i'd written and went back a step before row echelon form which has these equations:

    [itex] x + y + 5z = 5[/itex]
    [itex] y + z = 3[/itex]

    Giving a solution set of (4t-10, t, 3-t) but still this doesn't seem to be consistent. Any ideas where im going wrong? Thanks :)
  2. jcsd
  3. Apr 18, 2005 #2


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    You made a mistake reducing your matrix.

    You should get x = 0 , y = 5/3 z = 2/3.
  4. Apr 18, 2005 #3
    Hmm, are you sure? Because the question asks for the "general solutions" rather than any specific one. According to the way i reduced my matrix, there are infinitely many solutions. I'll have a go at typing out what ive done:


    R1 <-> R2

    [itex]\left(\begin{array}{ l c c | r } 1 & 1 & 5 & 5 \\ 3 & -2 & 5 & 0 \\ 1 & -2 & -1 & -4 \end{array}\right)[/itex]

    R2 -> R2 - 3R1
    R3 -> R3 - R1


    R2 -> -1/5 x R2
    R3 -> -1/3 x R3


    R3 -> R3 - R2
    R1 -> R1 - R2


    Sorry for the dodgy formatting..it doesnt seem to like me..hope that made sense though? If anyone knows how to stop it cutting off the top of the numbers then that would be great :)
    Last edited: Apr 18, 2005
  5. Apr 18, 2005 #4
    Well there's your problem. In your original post you dropped the coefficients for z.
    So you have x+3z =2, y+2z=3, z=t. So x=2-3t, y=3-2t, z=t.
  6. Apr 18, 2005 #5
    I cant believe i didnt see that :rofl: I knew i must've done something silly!! Thanks for pointing that stupid mistake out...i think i'll go crawl in a hole now :yuck: :biggrin:
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