Science:Math Exam Resources/Courses/MATH152/April 2017/Question A 23

MATH152 April 2017

  •  QA 1  •  QA 2  •  QA 3  •  QA 4  •  QA 5  •  QA 6  •  QA 7  •  QA 8  •  QA 9  •  QA 10  •  QA 11  •  QA 12  •  QA 13  •  QA 14  •  QA 15  •  QA 16  •  QA 17  •  QA 18  •  QA 19  •  QA 20  •  QA 21  •  QA 22  •  QA 23  •  QA 24  •  QA 25  •  QA 26  •  QA 27  •  QA 28  •  QA 29  •  QA 30  •  QB 1(a)  •  QB 1(b)  •  QB 1(c)  •  QB 2  •  QB 3(a)  •  QB 3(b)  •  QB 3(c)  •  QB 4(a)  •  QB 4(b)  •  QB 4(c)  •  QB 4(d)  •  QB 5(a)  •  QB 5(b)  •  QB 5(c)  •  QB 6(a)  •  QB 6(b)  •  QB 6(c)  •  

   Other MATH152 Exams

  •  April 2016  •  April 2015  •  April 2014  •  April 2022  •  April 2011  •  April 2010  •  April 2012  •  April 2013  •  April 2017  •  

Question A 23
Let $A$ and $B$ be matrices and let ${\mathbf {x}}$ be a vector. If $(A{\mathbf {x}})^{T}B$ is an $4\times 7$ matrix, is ${\mathbf {x}}$ a row vector or a column vector, and how many entries does it have? Justify your answer.

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!

Hint
For matrix multiplication the number of columns of the first matrix must be the same as the number of rows of the second one.

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.

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 can only multiply 2 matrices $X\times Y$ if the number of columns of $X$ is the same as the number of rows of $Y$. Thus if the product $(A{\mathbf {x}})^{T}B$ is a $4\times 7$ matrix, then we know $(A{\mathbf {x}})^{T}$ has size $4\times m$ and $B$ has size $m\times 7$, for some positive integer $m$. If the transposed matrix $(A{\mathbf {x}})^{T}$ is size $4\times m$, then the original matrix $A{\mathbf {x}}$ must be size $m\times 4$. This implies that $A$ is of size $m\times n$ and ${\mathbf {x}}$ is of size $n\times 4$ for some positive integers $m,n.$ Now, if ${\mathbf {x}}$ were a column vector, then $A{\mathbf {x}}$ would also be a column vector, of size $l\times 1$ for some $l$. However, since $4\neq 1$, we know that ${\mathbf {x}}$ must be a row vector. Finally, a row vector ${\mathbf {x}}$ is of size $1\times k$ for some $k$, and we know from our earlier reasoning ${\mathbf {x}}$ is also of size $n\times 4$. Thus, for our matrix operations to agree, we must have that $\color {blue}{\mathbf {x}}{\text{ is a }}1\times 4{\text{ row vector, with }}4{\text{ entries}}.$