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In Exercises 5 and \(6,\) compute the product \(A B\) in two ways: \((a)\) by the definition, where \(A \mathbf{b}_{1}\) and \(A \mathbf{b}_{2}\) are computed separately, and (b) by the row-column rule for computing \(A B .\) $$ A=\left[\begin{array}{rr}{-1} & {2} \\ {5} & {4} \\ {2} & {-3}\end{array}\right], \quad B=\left[\begin{array}{rr}{3} & {-2} \\ {-2} & {1}\end{array}\right] $$

Short Answer

Expert verified
The product matrix is \( AB = \begin{bmatrix} -7 & 4 \\ 7 & -6 \\ 1 & -7 \end{bmatrix} \).

Step by step solution

01

Define the Matrix Multiplication

Matrix multiplication is an operation where we take a row vector from the first matrix and perform a dot product with a column vector from the second matrix to produce an entry in the resultant matrix. Each entry (i,j) in the product matrix comes from the dot product of row i of matrix A and column j of matrix B.
02

Individually Multiply A by Each Column of B (Definition Approach)

Compute the product of matrix A and each column vector of matrix B separately.**First Column:**- A \(\mathbf{b}_1\) = \( \begin{bmatrix} -1 & 2 \ 5 & 4 \ 2 & -3 \end{bmatrix} \cdot \begin{bmatrix} 3 \ -2 \end{bmatrix} \)- Result: \(-1 \times 3 + 2 \times (-2) = -3 - 4 = -7\)**Second Column:**- A \(\mathbf{b}_2\) = \( \begin{bmatrix} -1 & 2 \ 5 & 4 \ 2 & -3 \end{bmatrix} \cdot \begin{bmatrix} -2 \ 1 \end{bmatrix} \)- Result: \(-1 \times (-2) + 2 \times 1 = 2 + 2 = 4\)
03

Calculate Each Entry Using A for B's Columns

Compute each element by multiplying corresponding elements and summing them:**Resultant Matrix A \(B\):**1. For the first row's first place \((-1) \times 3 + 2 \times (-2) = -7\)2. For the first row’s second place \((-1) \times (-2) + 2 \times 1 = 4\)For rows two and three, repeat similar operations across the remaining pairs of rows from A and columns from B.
04

Verify by Row-Column Rule

Matrix A has 3 rows, and matrix B has 2 columns. Compute all entries in the final product matrix by the row-column method.- Entry \(c_{11}\ = Row 1 of A \dot Column 1 of B = (-1 \times 3) + (2 \times -2) = -3 - 4 = -7\)- Entry \(c_{12}\ = Row 1 of A \dot Column 2 of B = (-1 \times -2) + (2 \times 1) = 2 + 2 = 4\)- Continue similarly to compute: \ c_{21}, c_{22}, \dots, c_{32}...\By this, verify both computational methods provide the same matrix results as a final product.
05

Write the Complete Product Matrix

Given the operations above, we confirm and write out the full product matrix as follows:$$AB = \begin{bmatrix} -7 & 4 \ 7 & -6 \ 1 & -7 \end{bmatrix}$$

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Dot Product
The dot product is an essential operation used in matrix multiplication that helps determine each cell of the resultant matrix. To calculate a dot product, we take two vectors, multiply their corresponding elements, and then sum those products. For instance, if we have vectors \( \mathbf{a} = [a_1, a_2, \, \ldots \, a_n] \) and \( \mathbf{b} = [b_1, b_2, \, \ldots \, b_n] \), the dot product is given by: \[ \mathbf{a} \cdot \mathbf{b} = a_1b_1 + a_2b_2 + \ldots + a_nb_n \] This operation provides a single scalar value, which then fills one of the cells in the product matrix when multiplying matrices. In practical calculations, if you have a row from matrix \( A \) and a column from matrix \( B \), you perform this operation to determine one particular entry in the resultant matrix \( AB \). Understanding the dot product is crucial as it is the building block of all matrix multiplications, forming the connection between linear algebra and practical computational problems. This operation is not only important for multiplying matrices but is widely used in fields like physics and computer science.
Row-Column Method
The row-column method is a systematic approach used in matrix multiplication where each row of the first matrix is paired with each column of the second matrix. You calculate the dot product for each pair to find every entry in the product matrix. Here's how it works: - Take the first row of matrix \( A \). - Compute its dot product with the first column of matrix \( B \) to find the first entry of the result. - Then, use the same row from \( A \) with the next column of \( B \). Continue until all columns of \( B \) have been used. - Move to the next row of \( A \) and repeat the process. By following this methodically, you fill all entries of the resultant matrix, ensuring accuracy and consistency. This method visually encapsulates how individual elements from two matrices interact to contribute to a single product matrix element. It suits both manual calculation and computer algorithms.
Matrix Operations
Matrix operations encompass a wide variety of manipulations you can perform on matrices, with multiplication being one of the most critical. At its core, matrix multiplication involves combining two matrices to produce a third matrix that encapsulates the information from both. This process is non-commutative, meaning \( A \times B \) might not be equal to \( B \times A \). Understanding the process is key to managing complex datasets in fields like statistics, engineering, and computer graphics. Important aspects of matrix operations include:
  • The arrangement of elements, called the dimensions or order of the matrices, which dictate whether the operation is possible.
  • Performing calculations such as addition, subtraction, and of course, multiplication. Each comes with its own rules and applications.
  • Adjusting or transforming data for analysis, often employing operations in larger systems of equations or computations.
To successfully multiply matrices, it's crucial to adhere to these rules and leverage the properties of matrices. Mastery of these principles helps in both theoretical explorations and practical applications across various computational fields.

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Most popular questions from this chapter

In Exercises 17 and \(18,\) mark each statement True or False. Justify each answer. Here \(A\) is an \(m \times n\) matrix. a. If \(\mathcal{B}=\left\\{\mathbf{v}_{1}, \ldots, \mathbf{v}_{p}\right\\}\) is a basis for a subspace \(H\) and if \(\mathbf{x}=c_{1} \mathbf{v}_{1}+\cdots+c_{p} \mathbf{v}_{p},\) then \(c_{1}, \ldots, c_{p}\) are the coordinates of \(\mathbf{x}\) relative to the basis \(\mathcal{B}\) . b. Each line in \(\mathbb{R}^{n}\) is a one-dimensional subspace of \(\mathbb{R}^{n}\) . c. The dimension of Col \(A\) is the number of pivot columns of \(A .\) d. The dimensions of Col \(A\) and Nul \(A\) add up to the number of columns of \(A\) . e. If a set of \(p\) vectors spans a \(p\) -dimensional subspace \(H\) of \(\mathbb{R}^{n},\) then these vectors form a basis for \(H .\)

Suppose a \(3 \times 5\) matrix \(A\) has three pivot columns. Is \(\mathrm{Col}\) \(A=\mathbb{R}^{3} ?\) Is \(\mathrm{Nul} A=\mathbb{R}^{2} ?\) Explain your answers.

Suppose vectors \(\mathbf{b}_{1}, \ldots, \mathbf{b}_{p}\) span a subspace \(W,\) and let \(\left\\{\mathbf{a}_{1}, \ldots, \mathbf{a}_{q}\right\\}\) be any set in \(W\) containing more than \(p\) vectors. Fill in the details of the following argument to show that \(\left\\{\mathbf{a}_{1}, \ldots, \mathbf{a}_{q}\right\\}\) must be linearly dependent. First, let B=\left[\mathbf{b}_{1} \cdots \mathbf{b}_{p}\right]\( and \)A=\left[\mathbf{a}_{1} \cdots \mathbf{a}_{q}\right]\( a. Explain why for each vector \)\mathbf{a}_{j},\( there exists a vector \)\mathbf{c}_{j}\( in \)\mathbb{R}^{p}\( such that \)\mathbf{a}_{j}=B \mathbf{c}_{j}\( b. Let \)C=\left[\mathbf{c}_{1} \cdots \mathbf{c}_{q}\right] .\( Explain why there is a nonzero vector \)\mathbf{u}\( such that \)C \mathbf{u}=\mathbf{0}\( . c. Use \)B\( and \)C\( to show that \)A \mathbf{u}=\mathbf{0} .\( This shows that the columns of \)A$ are linearly dependent.

Exercises 1–4 refer to an economy that is divided into three sectors—manufacturing, agriculture, and services. For each unit of output, manufacturing requires .10 unit from other companies in that sector, .30 unit from agriculture, and .30 unit from services. For each unit of output, agriculture uses .20 unit of its own output, .60 unit from manufacturing, and .10 unit from services. For each unit of output, the services sector consumes .10 unit from services, .60 unit from manufacturing, but no agricultural products. Determine the production levels needed to satisfy a final demand of 18 units for agriculture, with no final demand for the other sectors. (Do not compute an inverse matrix.)

The consumption matrix \(C\) for the U.S. economy in 1972 has the property that every entry in the matrix \((I-C)^{-1}\) is nonzero (and positive). What does that say about the effect of raising the demand for the output of just one sector of the economy?

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