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\begin{equation*} \begin{bmatrix} 4 & -1 & -1 & 0 &|&30 \\ -1 & 4 & 0 & -1&|&60 \\ -1 & 0 & 4 & -1&|&40 \\ 0 & -1 & -1 & 4&|&70 \end{bmatrix} \end{equation*}

What's the best way to solve the matrix above? There's a clear pattern of the diagonal 4's and 0's and the -1's so I feel like there has to be a better way of doing things rather than using scaling and row reduction.

If I do those methods I end up with messy fractions.

My Step 1:

New Row 2 = (1/4)Row 1 + Row 2

Even at step 1 I can tell the whole thing will be messy with fractions.

Is there a better way to solve this matrix? Or am I doing it wrong? Thanks.

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1 Answer 1

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Add all four rows to get $$\begin{pmatrix}2 &2 &2 &2\end{pmatrix}\begin{pmatrix}x_1\\x_2\\x_3\\x_4\end{pmatrix}= 200 $$

or

$$\begin{pmatrix}1 &1 &1 &1\end{pmatrix}\begin{pmatrix}x_1\\x_2\\x_3\\x_4\end{pmatrix}= 100 $$

Add that to every row to get $$\begin{pmatrix}5 &0 &0 &1\\ 0 & 5 & 1 & 0\\ 0 & 1 & 5 & 0\\ 1 & 0 & 0 & 5\end{pmatrix}\begin{pmatrix}x_1\\x_2\\x_3\\x_4\end{pmatrix}= \begin{pmatrix}130\\160\\140\\170\end{pmatrix} $$

Now you have two separated 2x2 systems. Can you take it from here?

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  • $\begingroup$ thanks, that's super interesting. is there a name for that process? $\endgroup$
    – Si Random
    Aug 27, 2020 at 13:03
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    $\begingroup$ I call it "sharply looking at the equation until you have an idea" :D No idea if there is a formal name for it. Also, can you please accept this answer? Then others will know that there is an viable answer here :) $\endgroup$
    – Nurator
    Aug 27, 2020 at 13:20
  • $\begingroup$ sure thing, i forgot i could do that cause i got more reputation now. also im still thinking this would be messy yea? cause like: new row 4 = row 1(-1/5) so then it'll be (-1/5) + 5 which is 4/5ths which is still messy, or is that just no other way, or am i doing it wrong $\endgroup$
    – Si Random
    Aug 27, 2020 at 13:25
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    $\begingroup$ you can do row1-5*row4, if you dislike fractions. But having one fraction and then multiplying the equation by 5/4 is not that bad. $\endgroup$
    – Nurator
    Aug 27, 2020 at 13:27
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    $\begingroup$ Wow thank you so much! I am really happy that an answer from here actually had a positive impact on someone in real life :) $\endgroup$
    – Nurator
    Sep 6, 2020 at 12:58

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