Problem 40 - Precalculus Mathematics for Calculus

Question

Solve the matrix equation by multiplying each side by the appropriate inverse matrix. $$\left[\begin{array}{rrr} 0 & -2 & 2 \\ 3 & 1 & 3 \\ 1 & -2 & 3 \end{array}\right]\left[\begin{array}{ll} x & u \\ y & v \\ z & w \end{array}\right]=\left[\begin{array}{lr} 3 & 6 \\ 6 & 12 \\ 0 & 0 \end{array}\right]$$

Step-by-Step Solution

Answer

Solve by finding \(A^{-1}\) and compute \(B = A^{-1}C\).

Step 1: Identify the Matrices

We are given the matrix equation: \(\left[\begin{array}{rrr} 0 & -2 & 2 \3 & 1 & 3 \1 & -2 & 3 \\end{array}\right]\left[\begin{array}{ll} x & u \y & v \z & w \\end{array}\right]=\left[\begin{array}{lr} 3 & 6 \6 & 12 \0 & 0 \\end{array}\right]\). Here, the first matrix is \(A\), the second matrix is \(B\), and the right-hand side is \(C\).

Step 2: Determine the Inverse of Matrix A

Matrix \(A = \left[\begin{array}{rrr} 0 & -2 & 2 \3 & 1 & 3 \1 & -2 & 3 \\end{array}\right] \). Calculate \(A^{-1}\) using the formula for the inverse of a 3x3 matrix: \(A^{-1} = \frac{1}{\det(A)} \text{adj}(A)\),where \(\text{adj}(A)\) is the adjugate of \(A\). Compute \(\det(A)\) and \(\text{adj}(A)\) to find \(A^{-1}\).

Step 3: Compute the Determinant of Matrix A

Calculate \(\det(A)\) using the formula for a 3x3 determinant:\(\det(A) = 0(1\cdot3 - 3\cdot-2) + 2(3\cdot3 - 3\cdot1) + 2(3\cdot-2 - 1\cdot3).\)Simplifying this will provide the determinant needed to find \(A^{-1}\).

Step 4: Calculate the Adjugate of Matrix A

To compute \( \text{adj}(A) \), find the cofactor matrix, and transpose it. Each element of the cofactor matrix is calculated by taking the determinant of the 2x2 sub-matrix that remains after removing the row and column of the element.

Step 5: Find the Inverse of Matrix A

After obtaining \(\det(A)\) and \(\text{adj}(A)\), find \(A^{-1}\) by calculating:\[ A^{-1} = \frac{1}{\det(A)} \cdot \text{adj}(A). \]

Step 6: Multiply Both Sides by A Inverse

Once \(A^{-1}\) is identified, solve the equation \[ A^{-1}A \cdot B = A^{-1}C \] to isolate matrix \(B\) on the left-hand side, hence: \[ B = A^{-1}C. \]

Step 7: Compute the Product A Inverse C

Calculate the product \(A^{-1}C\) to find the elements of matrix \(B\), giving you the solution matrix with elements \(x, y, z, u, v, w\).

Step 8: Verify the Solution

Substitute the calculated values back into the original matrix equation to ensure the equation holds true, confirming the solution is correct.

Key Concepts

Matrix Inverse

When dealing with systems of linear equations or matrix equations, the concept of a matrix inverse is crucial. Imagine a matrix as a sort of sophisticated multiplier, where an inverse resembles a reversal of that operation. Simply put, a matrix inverse is a matrix that, when multiplied with the original matrix, results in an identity matrix. The identity matrix operates like the number one in regular multiplication: it doesn't change the other number (or matrix) involved in the operation. Not every matrix has an inverse, and only square matrices (same number of rows and columns) can potentially have inverses.

For a matrix to have an inverse, it must be non-singular, meaning the determinant is not zero. If the determinant is zero, the matrix cannot be inverted. The process of finding the inverse of a matrix involves calculating its determinant and adjugate before applying the formula: \[ A^{-1} = \frac{1}{\det(A)} \cdot \text{adj}(A) \]Understanding how to find and use matrix inverses allows you to solve various linear equations more efficiently. To isolate a matrix in a matrix equation, multiply both sides by the inverse of a given matrix, effectively 'canceling out' the influence of that matrix on one side of the equation.

Determinant of a Matrix

The determinant is a special number which can be calculated from a square matrix. It plays a key role in understanding various properties of matrices. The determinant provides information about the matrix, such as whether it has an inverse. For a 3x3 matrix, the determinant can be complicated to compute, but it’s vital for finding both the inverse and adjugate.Here's the formula given a 3x3 matrix:\[ \text{If} \ A = \begin{bmatrix} a & b & c \ d & e & f \ g & h & i \end{bmatrix} , \ ext{then} \ \det(A) = a(ei - fh) - b(di - fg) + c(dh - eg) \]Breaking it down:

• First, multiply the element from the first row and its cofactor, this process needs to be repeated for each element of the first row.
• Substitute these into the determinant formula.
• Finally, simplify the result to get the value of the determinant.

The determinant tells you about the scaling factor of the transformation described by the matrix and is crucial when determining the invertibility of the matrix.

Adjugate Matrix

An adjugate matrix, or adjoint, is much like a "companion" to the original matrix. It is integral in computing the inverse of a matrix. The adjugate occurs through a process which involves replacing each element of a given matrix with its corresponding cofactor and then transposing the resulting matrix.The steps to find the adjugate matrix are as follows:

• First, determine the cofactor of each element of the matrix. A cofactor for a component is simply the signed determinant of the 2x2 matrix remaining after removing the component's row and column.
• Then, place these cofactors in a new matrix.
• Transpose this matrix, which involves swapping rows with columns.

The result is the adjugate matrix which alongside the determinant helps compute the inverse. When you tie it back with our main formula to find the inverse, \[ A^{-1} = \frac{1}{\det(A)} \cdot \text{adj}(A) \], the adjugate (or adj) matrix is vital to the equation. In essence, it complements the determinant to deliver the necessary inverse structure.