How to Add and Subtract Matrices? (+FREE Worksheet!)

How to Add and Subtract Matrices? (+FREE Worksheet!)

Adding and Subtracting Matrices – Example 1:

\(\begin{bmatrix} 2 & -5 & -3 \end{bmatrix}+ \begin{bmatrix}1 & -2 & -3 \end{bmatrix}\)

Tutor-style math help

Add and Subtract Matrices: what to notice and how to work it

Matrices skill
Matrix work starts with dimensions. The rows and columns tell you whether an operation is allowed before you calculate anything.

What to notice first

Write the matrix size first. Addition, multiplication, determinants, and inverses all have dimension rules.

Common student mistake

Do not multiply matrices entry by entry. Matrix multiplication uses each row of the first matrix with each column of the second.

Key formulas and cues

\((m\times n)+(m\times n)\text{ is allowed}\)
\((m\times n)(n\times p)=m\times p\)
\(\det\begin{pmatrix}a&b\\c&d\end{pmatrix}=ad-bc\)
\(A^{-1}A=I\)
[ 2 1 ][ 3 4 ] x [ 5 ][ 6 ] = rowdot

A reliable path

  1. Check dimensionsRows by columns determines what operation is legal.
  2. Use the correct ruleAddition is entry-by-entry; multiplication is row-by-column.
  3. Interpret the resultFor systems, translate the matrix answer back into variables.

Worked examples

Add matrices

Example: \([1\ 4]+[2\ 3]\)
  1. The matrices have the same size.
  2. Add matching entries.
  3. Compute each position.
Answer: \([3\ 7]\)

Multiplication size

Example: 2 by 3 matrix times 3 by 2 matrix
  1. Inner dimensions match: 3 and 3.
  2. The product is allowed.
  3. Outer dimensions give the result size.
Answer: 2 by 2 matrix
Try one before moving on
Try: What is the size of a 4×2 matrix times a 2×5 matrix?
Answer: 4×5.
Next step: do the matching worksheet or quiz while the method is still fresh, then come back and explain the first step in your own words.

Solution:

Add the elements in the matching positions: \(\begin{bmatrix} 2+1 & -5+(-2) & -3+(-3) \end{bmatrix}= \begin{bmatrix}3 & -7 & -6 \end{bmatrix}\)

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Adding and Subtracting Matrices – Example 2:

\(\begin{bmatrix}3 & 6 \\-1 & -3 \\-5 & -1\end{bmatrix}+ \begin{bmatrix}0 & -1 \\6 & 0 \\2 & 3\end{bmatrix} \)

Solution:

Add the elements in the matching positions: \(\begin{bmatrix}3+0 & 6+(-1) \\-1+6 & -3+0 \\-5+2 & -1+3\end{bmatrix} = \begin{bmatrix}3 & 5 \\5 & -3 \\-3 & 2\end{bmatrix} \)

Adding and Subtracting Matrices – Example 3:

\(\begin{bmatrix}2 & -1 \\0 & 1 \end{bmatrix}+\begin{bmatrix}3 & 1 \\5 & -2 \end{bmatrix}\)

Solution:

Add the elements in the matching positions: \(\begin{bmatrix}2+3 & -1+1 \\0+5 & 1+(-2) \end{bmatrix}=\begin{bmatrix}5 & 0 \\5 & -1 \end{bmatrix}\)

Adding and Subtracting Matrices – Example 4:

\(\begin{bmatrix}-1 & 2 \\3 & 4 \\0 & -2\end{bmatrix}- \begin{bmatrix}5 & -2 \\6 & 1 \\-3 & 4\end{bmatrix} \)

Solution:

Subtract the elements in the matching positions: \(\begin{bmatrix}-1-5 & 2-(-2) \\3-6 & 4-1 \\0-(-3) & -2-4\end{bmatrix}= \begin{bmatrix}-6 & 4 \\-3 & 3 \\3 & -6\end{bmatrix}\)

Exercises for Adding and Subtracting Matrices

  1. \(\color{blue}{\begin{bmatrix}6 & 8 \\-14 & 33 \end{bmatrix}-\begin{bmatrix}12 & 5 \\-27 & -8 \end{bmatrix}}\)
  2. \(\color{blue}{\begin{bmatrix}12 & 21 \\-17 & 33 \end{bmatrix}-\begin{bmatrix}5& -8 \\2 & 19 \end{bmatrix}}\)
  3. \(\color{blue}{\begin{bmatrix}-5 & 2& -2 \\4 & -2&0 \end{bmatrix}-\begin{bmatrix}6 & -5&-6 \\1 & 3&-3 \end{bmatrix}}\)
  4. \(\color{blue}{\begin{bmatrix}-6r+t \\-r\\6s \end{bmatrix}+\begin{bmatrix}6r \\-4t\\-3r+2 \end{bmatrix}}\)
  5. \(\color{blue}{\begin{bmatrix}16 & -4 \\-38 & 24 \end{bmatrix}+\begin{bmatrix}9 & -6 \\5& 2 \end{bmatrix}}\)
  6. \(\color{blue}{\begin{bmatrix}18 & -5 \\-32 & 14 \end{bmatrix}+\begin{bmatrix}11& -6 \\7 & 2 \end{bmatrix}}\)
  7. \(\color{blue}{\begin{bmatrix}-4n & n+m \\-2n & -4m\end{bmatrix}+\begin{bmatrix}4 & -5 \\3 & 0 \end{bmatrix}}\)
  8. \(\color{blue}{\begin{bmatrix}z-5 \\-6\\-1-6z\\3y \end{bmatrix}+\begin{bmatrix}-3y\\3z\\5+z\\4z \end{bmatrix}}\)

Solve.

  1. \(\color{blue}{\begin{bmatrix}-6 & 3 \\13 & 41 \end{bmatrix}}\)
  2. \(\color{blue}{\begin{bmatrix}7 & 29 \\-19 & 14 \end{bmatrix}}\)
  3. \(\color{blue}{\begin{bmatrix}-11 & 7&4 \\3 & -5&3 \end{bmatrix}}\)
  4. \(\color{blue}{\begin{bmatrix}t \\-r-4t\\6s-3r+2 \end{bmatrix}} \)
  5. \(\color{blue}{\begin{bmatrix}25 & -10 \\-33 & 26 \end{bmatrix}}\)
  6. \(\color{blue}{\begin{bmatrix}29 & -11 \\-25 & 16 \end{bmatrix}}\)
  7. \(\color{blue}{\begin{bmatrix}-4n+4 & n+m-5 \\-2n+3 & -4m\end{bmatrix}}\)
  8. \(\color{blue}{\begin{bmatrix}z-5-3y \\-6+3z\\4-5z\\3y+4z \end{bmatrix}} \)

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