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Given the classical adjoint matrix and the determinant, how to find the inverse matrix?

Once the classical adjoint matrix (adjAadj A) and the determinant (det⁡(A)\det(A)) are known, the inverse matrix A−1A^{-1} is found by multiplying the adjugate matrix by the reciprocal of the determinant. The formula is A−1=1det⁡(A)adjAA^{-1}=\frac{1}{\det(A)}adj A. This requires that det⁡(A)≠0\det(A)\neq 0.

Conditions

  • A is a square matrix.
  • det⁡(A)≠0\det(A)\neq 0.
  • The classical adjoint matrix adjAadj A has been calculated.

Reasoning, step by step

  1. Verify that det⁡(A)≠0\det(A)\neq 0.
  2. Calculate the reciprocal of the determinant: 1det⁡(A)\frac{1}{\det(A)}.
  3. Multiply every entry of the adjugate matrix adjAadj A by this scalar value.
  4. The resulting matrix is the inverse matrix $A−1A^{-1}.

Example

In the video, det⁡(A)=−1\det(A)=-1 and adjA=[−148117−10−119−11−1]adj A=\begin{bmatrix}-14&8&1\\17&-10&-1\\19&-11&-1\end{bmatrix}. The inverse is calculated as A−1=1−1[−148117−10−119−11−1]=[14−8−1−17101−19111]A^{-1}=\frac{1}{-1}\begin{bmatrix}-14&8&1\\17&-10&-1\\19&-11&-1\end{bmatrix} = \begin{bmatrix}14&-8&-1\\-17&10&1\\-19&11&1\end{bmatrix}.

Common misconceptions

  • Forgetting to divide by the determinant and assuming the adjugate matrix is the inverse.
  • Dividing the determinant by the adjugate matrix instead of the other way around.
  • Applying the scalar multiplication incorrectly to only some entries of the adjugate matrix.

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