Common Core: High School - Number and Quantity : Zero and Identity Matrices: CCSS.Math.Content.HSN-VM.C.10

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Example Questions

Example Question #11 : Zero And Identity Matrices: Ccss.Math.Content.Hsn Vm.C.10

Find the inverse of the following matrix.

\(\displaystyle \begin{bmatrix} -2&-3 \\ 2& 0 \end{bmatrix}\)

Possible Answers:

\(\displaystyle \begin{bmatrix} 0&-\frac{1}{2} \\ -\frac{1}{3}& -\frac{1}{3} \end{bmatrix}\)

\(\displaystyle \begin{bmatrix} 0&\frac{1}{2} \\ -\frac{1}{3}& -\frac{1}{3} \end{bmatrix}\)

\(\displaystyle \begin{bmatrix} 0&\frac{1}{2} \\ \frac{1}{3}& -\frac{1}{3} \end{bmatrix}\)

\(\displaystyle \begin{bmatrix} 0&\frac{1}{2} \\ \frac{1}{3}& \frac{1}{3} \end{bmatrix}\)

Correct answer:

\(\displaystyle \begin{bmatrix} 0&\frac{1}{2} \\ -\frac{1}{3}& -\frac{1}{3} \end{bmatrix}\)

Explanation:

In order to find the inverse of a matrix, we need to recall the formula for finding an inverse of a 2x2 matrix. 

\(\displaystyle A^{-1}=\frac{1}{a\cdot d-b\cdot c}\begin{bmatrix} d&-b \\ -c& a \end{bmatrix}\), where \(\displaystyle a,b,c,d\) refer to position within the general 2x2 matrix \(\displaystyle \begin{bmatrix} a& b\\ c&d \end{bmatrix}\).

 

The first step is to figure out what the fraction is.

\(\displaystyle \begin{bmatrix} -2&-3 \\ 2& 0 \end{bmatrix}\)

In this case \(\displaystyle a=-2\)\(\displaystyle b=-3\)\(\displaystyle c=2\), and \(\displaystyle d=0\).

\(\displaystyle \frac{1}{a\cdot d-b\cdot c}=\frac{1}{(-2)\cdot0-(-3)\cdot2}=\frac{1}{0+6}=\frac{1}{6}\)

 

The next step is to swap the off diagonal entries, and the multiply by negative 1 on the off diagonal entries.

\(\displaystyle \begin{bmatrix} -2&-3 \\ 2& 0 \end{bmatrix}\rightarrow\) \(\displaystyle \begin{bmatrix} 0&3 \\ -2&-2 \end{bmatrix}\)

 

The last step is to multiply them together.

\(\displaystyle \frac{1}{6}\)  \(\displaystyle \begin{bmatrix} 0&3 \\ -2&-2 \end{bmatrix}\) 

 

\(\displaystyle =\begin{bmatrix} 0\cdot\frac{1}{6}&3\cdot\frac{1}{6} \\ -2\cdot\frac{1}{6}& -2\cdot\frac{1}{6} \end{bmatrix}\)

\(\displaystyle =\begin{bmatrix} 0&\frac{1}{2} \\ -\frac{1}{3}& -\frac{1}{3} \end{bmatrix}\)

Example Question #252 : High School: Number And Quantity

Find the inverse of the following matrix.

\(\displaystyle \begin{bmatrix} 5&-3 \\ -3& 0 \end{bmatrix}\)

Possible Answers:

\(\displaystyle \begin{bmatrix} 0&-\frac{1}{3} \\ \frac{1}{3}& \frac{5}{9} \end{bmatrix}\)

\(\displaystyle \begin{bmatrix} 0&-\frac{1}{3} \\ -\frac{1}{3}& -\frac{5}{9} \end{bmatrix}\)

\(\displaystyle \begin{bmatrix} 0&-\frac{1}{3} \\ -\frac{1}{3}& \frac{5}{9} \end{bmatrix}\)

\(\displaystyle \begin{bmatrix} 0&\frac{1}{3} \\ \frac{1}{3}& \frac{5}{9} \end{bmatrix}\)

Correct answer:

\(\displaystyle \begin{bmatrix} 0&-\frac{1}{3} \\ -\frac{1}{3}& -\frac{5}{9} \end{bmatrix}\)

Explanation:

In order to find the inverse of a matrix, we need to recall the formula for finding an inverse of a 2x2 matrix. 

\(\displaystyle A^{-1}=\frac{1}{a\cdot d-b\cdot c}\begin{bmatrix} d&-b \\ -c& a \end{bmatrix}\), where \(\displaystyle a,b,c,d\) refer to position within the general 2x2 matrix \(\displaystyle \begin{bmatrix} a& b\\ c&d \end{bmatrix}\).

 

The first step is to figure out what the fraction is.

\(\displaystyle \begin{bmatrix} 5&-3 \\ -3& 0 \end{bmatrix}\)

In this case \(\displaystyle a=5\)\(\displaystyle b=-3\)\(\displaystyle c=-3\), and \(\displaystyle d=0\).

\(\displaystyle \frac{1}{a\cdot d-b\cdot c}=\frac{1}{5\cdot0-(-3)\cdot(-3)}=\frac{1}{0-9}=-\frac{1}{9}\)

 

The next step is to swap the off diagonal entries, and the multiply by negative 1 on the off diagonal entries.

\(\displaystyle \begin{bmatrix} 5&-3 \\ -3& 0 \end{bmatrix}\rightarrow\) \(\displaystyle \begin{bmatrix} 0&3 \\3&5 \end{bmatrix}\)

 

The last step is to multiply them together.

\(\displaystyle -\frac{1}{9}\)  \(\displaystyle \begin{bmatrix} 0&3 \\3&5 \end{bmatrix}\) 

 

\(\displaystyle =\begin{bmatrix} 0\cdot-\frac{1}{9}&3\cdot-\frac{1}{9} \\ 3\cdot-\frac{1}{9}& 5\cdot-\frac{1}{9} \end{bmatrix}\)

\(\displaystyle =\begin{bmatrix} 0&-\frac{1}{3} \\ -\frac{1}{3}& -\frac{5}{9} \end{bmatrix}\)

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