Science:Math Exam Resources/Courses/MATH152/April 2012/Question 02 (a)

MATH152 April 2012

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[hide] Question 02 (a)
Consider the following system of linear equations ${\begin{aligned}2x-y+2z&=a\\x+3y-6z&=b\\3x+2y-4z&=c,\end{aligned}}$ where a,b, and c $\in \mathbb {R}$. Write the system in matrix form $A\mathbf {x} =\mathbf {b}$. Clearly define A, $\mathbf {b}$, and $\mathbf {x}$.

Make sure you understand the problem fully: What is the question asking you to do? Are there specific conditions or constraints that you should take note of? How will you know if your answer is correct from your work only? Can you rephrase the question in your own words in a way that makes sense to you?

If you are stuck, check the hint below. Consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it!

[show] Hint
Write out a general linear 3x3 system ${\begin{bmatrix}a_{11}&a_{12}&a_{13}\\a_{21}&a_{22}&a_{23}\\a_{31}&a_{32}&a_{33}\end{bmatrix}}{\begin{bmatrix}x_{1}\\x_{2}\\x_{3}\end{bmatrix}}={\begin{bmatrix}b_{1}\\b_{2}\\b_{3}\end{bmatrix}}.$ Compare this to what is given in the question.

Checking a solution serves two purposes: helping you if, after having used the hint, you still are stuck on the problem; or if you have solved the problem and would like to check your work. If you are stuck on a problem: Read the solution slowly and as soon as you feel you could finish the problem on your own, hide it and work on the problem. Come back later to the solution if you are stuck or if you want to check your work. If you want to check your work: Don't only focus on the answer, problems are mostly marked for the work you do, make sure you understand all the steps that were required to complete the problem and see if you made mistakes or forgot some aspects. Your goal is to check that your mental process was correct, not only the result.

[show] Solution

The matrix $\displaystyle A$ should be composed of the coefficients of the variables on the left hand sides of the equations. Thus $A={\begin{bmatrix}2&-1&2\\1&3&-6\\3&2&-4\end{bmatrix}}$ Next, we note that to compose the $\mathbf {x}$ on the right to the matrix $\displaystyle A$. The right hand side has to be $\mathbf {b}$. Thus $\mathbf {x} ={\begin{bmatrix}x\\y\\z\end{bmatrix}}\quad \quad \mathbf {b} ={\begin{bmatrix}a\\b\\c\end{bmatrix}}$ As a sanity check, perform the matrix multiplication so that you can verify that this works.

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