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When finding the inverse matrix using the classical adjoint matrix, what is the first step?

The first step is to calculate the determinant of the matrix, det⁡(A)\det(A), and verify that it is non-zero. This check is essential because the formula for the inverse, A−1=1det⁡(A)adjAA^{-1}=\frac{1}{\det(A)}adj A, requires division by the determinant. If det⁡(A)=0\det(A)=0, the inverse does not exist, and further calculation of the adjugate matrix is unnecessary.

Conditions

  • The goal is to find the inverse matrix using the classical adjoint method.
  • A is a square matrix.

Reasoning, step by step

  1. Calculate the determinant of matrix A.
  2. Check if det⁡(A)≠0\det(A) \neq 0.
  3. If det⁡(A)=0\det(A) = 0, stop; the matrix is singular and has no inverse.
  4. If det⁡(A)≠0\det(A) \neq 0, proceed to calculate the minor determinants and construct the adjugate matrix.

Example

The video begins by expanding along the first row to find det⁡(A)=−1\det(A)=-1. Since −1≠0-1 \neq 0, the instructor confirms the inverse exists and proceeds to calculate the minors.

Common misconceptions

  • Starting to calculate the cofactors or adjugate matrix before checking the determinant.
  • Assuming that every square matrix has an inverse.
  • Forgetting that a zero determinant means the division in the inverse formula is undefined.

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Answers are generated from source material and independently checked. Consult the original video or creator if something is unclear.