Solution of a system of linear equations

Question

I tried to solve a system of three linear equations in three unknowns. After a series of elementary row operations the augmented matrix of the system of linear equations becomes

$$\left(\begin{array}{ccc|c}1 & 0 & 0 & 3 \\0 & 1 & 0 & 2 \\0 & 0 & 1 & 3\end{array}\right).$$

Next, I wrote:

"The above matrix represents the following new system of linear equations

\begin{align} x_1&=3\\ x_2&=2\\ x_3&=3.\\ \end{align}

Since the determinant of the coefficient matrix of the above new system of linear equations is not zero, the original linear system has a unique solution. Thus, the unique solution of the original system is $x_1=3, x_2=2, x_3=3$."

In the solution, can we directly go to the final answer (Thus, the unique solution of the original system of linear equations is $x_1=3, x_2=2, x_3=3$.) without giving the poof that the original linear system has a unique solution (by skipping the sentence ‘Since the determinant of the coefficient matrix of the corresponding linear system of the resultant augmented matrix is not zero, the original linear system has a unique solution.’)?

Answer 1

I became aware of this question by way of an answer on Meta and feel I must push back against the comment of DonAntonio and the answer of amWhy.

Uniqueness of the solution of the system $$ \begin{aligned} x_1&=3\\ x_2&=2\\ x_3&=3 \end{aligned} $$ is obvious and needs no proof. What is there to prove? Is it conceivable that if you plug in numbers other than $3,$ $2,$ and $3$ for $x_1,$ $x_2,$ and $x_3$ you might obtain three true statements?

The determinant is a complicated object, and by bringing it in in this situation, you are making something simple appear much more difficult than it actually is.

Here's what I think you were probably getting at: Let's start with a simpler analogue. Is the solution of the equation $x=2$ unique? Of course is is: $2$ is the only solution. Now $x=2$ may be the end result of simplifying a more complicated equation, such as $13x=26.$ The latter is a special case of the general equation $ax=b.$ It is certainly the case that the latter has a unique solution if and only if $a\ne0.$ If $a=0,$ then there is no solution unless $b=0,$ in which case there are infinitely many solutions.

Likewise, the matrix equation $Ax=b,$ where $A$ is a square matrix and $x$ and $b$ are column vectors, has a unique solution if and only if $\det A\ne0.$ If $\det A=0,$ then it has either no solution or infinitely many solutions.

So it is helpful to introduce the determinant to make statements about the nature of the solution set of the general equation $Ax=b.$ But for concrete $A$ and $b,$ it is usually more efficient to row reduce the system than to compute $\det A.$ (More precisely, computing $\det A$ is best done by actually performing row reduction, but there is no need to mention determinants if you are row reducing to solve a concrete problem.) The end result of the row-reduction process will tell you whether there is a unique solution or not.

The only thing that might need proof is that the three row operations (swapping rows, multiplying a row by a non-zero number, adding a multiple of one row to another row) preserve the solution set. That is generally proved in a linear algebra course, and you can probably assume it from that point on. If not, let $S$ be a system and let $S'$ be the system that results from applying a row operation. You just need to prove that any solution to $S$ is a solution to $S',$ and that any solution to $S',$ is a solution to $S.$ This is straightforward, but it seems like overkill to do it in every row reduction problem you perform.

Answer 2

I would suggest that no: you shouldn't skip the explanation. You should explain why you know the solution to the original equation is unique. This doesn't necessarily mean a full blown proof, but a comment, as you included, demonstrating that you have a thorough understanding as to "why."

Why?

• To engrave this information/explanation in your memory and understanding.
• To "cover all bases" in terms of ensuring your answer has all the necessary justification required by your instructor.

In the end, it can't hurt to include that justification, and leaving it out could result in not getting full credit.

But before you have to take any time-limited exams on the subject matter, be sure to consult your instructor. When first learning about solutions to linear systems of equations, it is almost always desirable to include your justification. Down the road, that information might well be considered unnecessary. Clear this up with your instructor.