Question 48 of 58advanced🔧 ApplyLong Answer5 marks

If A = [2−3532−411−2]\begin{bmatrix} 2 & -3 & 5 \\ 3 & 2 & -4 \\ 1 & 1 & -2 \end{bmatrix}, find A−1A^{-1}. Using A−1A^{-1} solve the system of equations 2x - 3y + 5z = 11 3x + 2y - 4z = -5 x + y - 2z = -3

Correct Answer

The system of equations can be written in matrix form as AX = B, where A = [2−3532−411−2]\begin{bmatrix} 2 & -3 & 5 \\ 3 & 2 & -4 \\ 1 & 1 & -2 \end{bmatrix}, X = [xyz]\begin{bmatrix} x \\ y \\ z \end{bmatrix}, B = [11−5−3]\begin{bmatrix} 11 \\ -5 \\ -3 \end{bmatrix}

Step 1: Find |A| |A| = 2[(2)(-2) - (-4)(1)] - (-3)[(3)(-2) - (-4)(1)] + 5[(3)(1) - (2)(1)] = 2(0) + 3(-2) + 5(1) = -1 ≠\neq 0 Since |A| ≠\neq 0, A−1A^{-1} exists.

Step 2: Find cofactors A_{11} = 0, A_{12} = 2, A_{13} = 1 A_{21} = -1, A_{22} = -9, A_{23} = -5 A_{31} = 2, A_{32} = 23, A_{33} = 13

Step 3: Find adj(A) and A−1A^{-1} adj(A) = [0−122−9231−513]\begin{bmatrix} 0 & -1 & 2 \\ 2 & -9 & 23 \\ 1 & -5 & 13 \end{bmatrix} A−1A^{-1} = (1/|A|) × adj(A) = [01−2−29−23−15−13]\begin{bmatrix} 0 & 1 & -2 \\ -2 & 9 & -23 \\ -1 & 5 & -13 \end{bmatrix}

Step 4: Solve for X X = A−1A^{-1}B = [01−2−29−23−15−13][11−5−3]=[0−5+6−22−45+69−11−25+39]\begin{bmatrix} 0 & 1 & -2 \\ -2 & 9 & -23 \\ -1 & 5 & -13 \end{bmatrix} \begin{bmatrix} 11 \\ -5 \\ -3 \end{bmatrix} = \begin{bmatrix} 0-5+6 \\ -22-45+69 \\ -11-25+39 \end{bmatrix} = [123]\begin{bmatrix} 1 \\ 2 \\ 3 \end{bmatrix}

Hence, x = 1, y = 2, z = 3.

Exercise: EXERCISE 4.5 | Q: 15 | (Page 23)
For More Understanding

Explanation

This question follows the standard matrix method for solving a system of linear equations, as demonstrated in Example 17 of the textbook. First, we verify that the coefficient matrix A is nonsingular by checking |A| ≠\neq 0. Then we find the inverse using the formula A−1A^{-1} = (1/|A|) × adj(A), where adj(A) is the transpose of the cofactor matrix. Finally, we use X = A−1A^{-1}B to find the solution.

Solution Steps

  1. Step 1: Calculate determinant |A| = -1 ≠\neq 0, confirming A−1A^{-1} exists

  2. Step 2: Calculate all 9 cofactors of matrix A

  3. Step 3: Form adj(A) by transposing cofactor matrix, then find A−1A^{-1}

  4. Step 4: Multiply A−1A^{-1} with B to get X = [1, 2, 3]^T