Add and Subtract Matrices
Learning Objectives
- Determine the dimensions of a matrix
- Add and subtract two matrices
[latex]A=\left[\begin{array}{cc}1& 2\\ 3& 4\end{array}\right],B=\left[\begin{array}{ccc}1& 2& 7\\ 0& -5& 6\\ 7& 8& 2\end{array}\right],C=\left[\begin{array}{c}-1\\ 0\\ 3\end{array}\begin{array}{c}3\\ 2\\ 1\end{array}\right][/latex]
Describing Matrices
A matrix is often referred to by its size or dimensions: [latex]\text{ }m\text{ }\times \text{ }n\text{ }[/latex] indicating [latex]m[/latex] rows and [latex]n[/latex] columns. Matrix entries are defined first by row and then by column. For example, to locate the entry in matrix [latex]A[/latex] identified as [latex]{a}_{ij},\text{}[/latex] we look for the entry in row [latex]i,\text{}[/latex] column [latex]j[/latex]. In matrix [latex]A\text{, \hspace{0.17em}}[/latex] shown below, the entry in row 2, column 3 is [latex]{a}_{23}[/latex].[latex]A=\left[\begin{array}{ccc}{a}_{11}& {a}_{12}& {a}_{13}\\ {a}_{21}& {a}_{22}& {a}_{23}\\ {a}_{31}& {a}_{32}& {a}_{33}\end{array}\right][/latex]
A square matrix is a matrix with dimensions [latex]\text{ }n\text{ }\times \text{ }n,\text{}[/latex] meaning that it has the same number of rows as columns. The [latex]3\times 3[/latex] matrix above is an example of a square matrix. A row matrix is a matrix consisting of one row with dimensions [latex]1\text{ }\times \text{ }n[/latex].[latex]\left[\begin{array}{ccc}{a}_{11}& {a}_{12}& {a}_{13}\end{array}\right][/latex]
A column matrix is a matrix consisting of one column with dimensions [latex]m\text{ }\times \text{ }1[/latex].[latex]\left[\begin{array}{c}{a}_{11}\\ {a}_{21}\\ {a}_{31}\end{array}\right][/latex]
A matrix may be used to represent a system of equations. In these cases, the numbers represent the coefficients of the variables in the system. Matrices often make solving systems of equations easier because they are not encumbered with variables. We will investigate this idea further in the next section, but first we will look at basic matrix operations.A General Note: Matrices
A matrix is a rectangular array of numbers that is usually named by a capital letter: [latex]A,B,C,\text{}[/latex] and so on. Each entry in a matrix is referred to as [latex]{a}_{ij}[/latex], such that [latex]i[/latex] represents the row and [latex]j[/latex] represents the column. Matrices are often referred to by their dimensions: [latex]m\times n[/latex] indicating [latex]m[/latex] rows and [latex]n[/latex] columns.Example: Finding the Dimensions of the Given Matrix and Locating Entries
Given matrix [latex]A:[/latex]- What are the dimensions of matrix [latex]A?[/latex]
- What are the entries at [latex]{a}_{31}[/latex] and [latex]{a}_{22}?[/latex]
[latex]A=\left[\begin{array}{rrrr}\hfill 2& \hfill & \hfill 1& \hfill 0\\ \hfill 2& \hfill & \hfill 4& \hfill 7\\ \hfill 3& \hfill & \hfill 1& \hfill -2\end{array}\right][/latex]
Answer:
- The dimensions are [latex]\text{ }3\text{ }\times \text{ }3\text{ }[/latex] because there are three rows and three columns.
- Entry [latex]{a}_{31}[/latex] is the number at row 3, column 1, which is 3. The entry [latex]{a}_{22}[/latex] is the number at row 2, column 2, which is 4. Remember, the row comes first, then the column.
Adding and Subtracting Matrices
We use matrices to list data or to represent systems. Because the entries are numbers, we can perform operations on matrices. We add or subtract matrices by adding or subtracting corresponding entries. In order to do this, the entries must correspond. Therefore, addition and subtraction of matrices is only possible when the matrices have the same dimensions. We can add or subtract a [latex]\text{ }3\text{ }\times \text{ }3\text{ }[/latex] matrix and another [latex]\text{ }3\text{ }\times \text{ }3\text{ }[/latex] matrix, but we cannot add or subtract a [latex]\text{ }2\text{ }\times \text{ }3\text{ }[/latex] matrix and a [latex]\text{ }3\text{ }\times \text{ }3\text{ }[/latex] matrix because some entries in one matrix will not have a corresponding entry in the other matrix.A General Note: Adding and Subtracting Matrices
Given matrices [latex]A[/latex] and [latex]B[/latex] of like dimensions, addition and subtraction of [latex]A[/latex] and [latex]B[/latex] will produce matrix [latex]C[/latex] or matrix [latex]D[/latex] of the same dimension.[latex]A+B=C\text{ such that }{a}_{ij}+{b}_{ij}={c}_{ij}[/latex]
[latex]A-B=D\text{ such that }{a}_{ij}-{b}_{ij}={d}_{ij}[/latex]
Matrix addition is commutative.[latex]A+B=B+A[/latex]
It is also associative.[latex]\left(A+B\right)+C=A+\left(B+C\right)[/latex]
Example: Finding the Sum of Matrices
Find the sum of [latex]A[/latex] and [latex]B,\text{}[/latex] given[latex]A=\left[\begin{array}{cc}a& b\\ c& d\end{array}\right]\text{ and }B=\left[\begin{array}{cc}e& f\\ g& h\end{array}\right][/latex]
Answer: Add corresponding entries.
[latex]\begin{array}{l}A+B=\left[\begin{array}{cc}a& b\\ c& d\end{array}\right]+\left[\begin{array}{cc}e& f\\ g& h\end{array}\right]\hfill \\ \text{ }=\left[\begin{array}{ccc}a+e& & b+f\\ c+g& & d+h\end{array}\right]\hfill \end{array}[/latex]
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