Questions: Find the inverse of the matrix (if it exists). (If an answer does not exist, enter DNE.)
[1 2
5 9]
Transcript text: Find the inverse of the matrix (if it exists). (If an answer does not exist, enter DNE.)
\[
\left[\begin{array}{ll}
1 & 2 \\
5 & 9
\end{array}\right]
\]
Solution
Solution Steps
To find the inverse of a matrix, we need to check if the determinant of the matrix is non-zero. If the determinant is zero, the inverse does not exist (DNE). If the determinant is non-zero, we can use the formula for the inverse of a 2x2 matrix.
Step 1: Define the Matrix
We are given the matrix:
\[
\begin{bmatrix}
1 & 2 \\
5 & 9
\end{bmatrix}
\]
Step 2: Calculate the Determinant
The determinant of a \(2 \times 2\) matrix \(\begin{bmatrix} a & b \\ c & d \end{bmatrix}\) is calculated as:
\[
\text{det} = ad - bc
\]
For our matrix:
\[
\text{det} = (1 \cdot 9) - (2 \cdot 5) = 9 - 10 = -1
\]
Step 3: Check if the Determinant is Non-Zero
Since \(\text{det} = -1 \neq 0\), the inverse of the matrix exists.
Step 4: Calculate the Inverse
The inverse of a \(2 \times 2\) matrix \(\begin{bmatrix} a & b \\ c & d \end{bmatrix}\) is given by:
\[
\text{inverse} = \frac{1}{\text{det}} \begin{bmatrix} d & -b \\ -c & a \end{bmatrix}
\]
For our matrix:
\[
\text{inverse} = \frac{1}{-1} \begin{bmatrix} 9 & -2 \\ -5 & 1 \end{bmatrix} = \begin{bmatrix} -9 & 2 \\ 5 & -1 \end{bmatrix}
\]
Final Answer
The inverse of the matrix is:
\[
\boxed{
\begin{bmatrix}
-9 & 2 \\
5 & -1
\end{bmatrix}
}
\]