Science:Math Exam Resources/Courses/MATH152/April 2013/Question A 04

MATH152 April 2013

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Question A 04
Consider the matrix $M$ given by ${\begin{bmatrix}1&7&3\\0&p&5\\1&pq+7&8\end{bmatrix}}.$ For what $p$ and $q$ does the matrix $M$ have rank 2?

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 hints below. Read the first one and consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it! If after a while you are still stuck, go for the next hint.

Hint 1
What does it mean for the matrix to have rank 2? How might computing its row echelon form help you?

Hint 2
For a rank 2 matrix in row echelon form, the first two rows must each have at least 1 nonzero entry, while the last row must consist of zeroes only.

Checking a solution serves two purposes: helping you if, after having used all the hints, 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.

Solution

Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies. We reduce the matrix to row echelon form. Subtracting the first row from the last row transforms $M={\begin{bmatrix}1&7&3\\0&p&5\\1&pq+7&8\end{bmatrix}}$ into ${\begin{bmatrix}1&7&3\\0&p&5\\0&pq&5\end{bmatrix}}.$ Next, subtracting $q$ times the second row from the third row yields ${\begin{bmatrix}1&7&3\\0&p&5\\0&0&5-5q\end{bmatrix}}.$ Clearly, this matrix has rank at least 2 (since, for instance, the first and third columns are linearly independent regardless of the value of $q$). On the other hand, it must have rank less than 3 whenever $p=0$ or $5-5q=0$ (or both), which is to say that $M$ has rank 2 exactly when $\color {blue}p=0$ or $\color {blue}q=1$ (or both).

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