The adjoint of a matrix, also known as the adjugate, is a matrix that is related to the original matrix and is used in various mathematical operations, including finding the inverse of a matrix. To find the adjoint of a matrix, you can follow these steps:
Determine the Matrix of Cofactors: Start by finding the cofactor matrix of the given matrix. To do this, calculate the cofactor of each element in the original matrix. The cofactor of an element in the i-th row and j-th column (denoted as C[i][j]) is the determinant of the submatrix obtained by deleting the i-th row and j-th column, multiplied by (-1)^(i+j).
For example, if you have a 3×3 matrix A, the cofactor C[i][j] of the element in the i-th row and j-th column is given by:
C[i][j] = (-1)^(i+j) * Det(Submatrix obtained by deleting the i-th row and j-th column)
Transpose the Cofactor Matrix: Once you have calculated all the cofactors, transpose the resulting matrix. Transposing a matrix means swapping its rows and columns.
The Transposed Matrix is the Adjoint: The transposed matrix from step 2 is the adjoint of the original matrix.
Here’s a step-by-step example:
Let’s say you have a 3×3 matrix A:
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A = | 1 2 3 | | 4 5 6 | | 7 8 9 |
Calculate the cofactors for each element:
C[1][1] = Det(Submatrix without the 1st row and 1st column) = Det(|5 6|) = 5
C[1][2] = Det(Submatrix without the 1st row and 2nd column) = Det(|4 6|) = -6
C[1][3] = Det(Submatrix without the 1st row and 3rd column) = Det(|4 5|) = 5
C[2][1] = Det(Submatrix without the 2nd row and 1st column) = Det(|8 9|) = 8
C[2][2] = Det(Submatrix without the 2nd row and 2nd column) = Det(|7 9|) = -9
C[2][3] = Det(Submatrix without the 2nd row and 3rd column) = Det(|7 8|) = 7
C[3][1] = Det(Submatrix without the 3rd row and 1st column) = Det(|2 3|) = -3
C[3][2] = Det(Submatrix without the 3rd row and 2nd column) = Det(|1 3|) = 3
C[3][3] = Det(Submatrix without the 3rd row and 3rd column) = Det(|1 2|) = -1
Transpose the matrix of cofactors:
