Let
What is the contactor of the entry A23 = 5
The co-factor of 5 is obtained by eliminating the row and column of 5. By doing this, we obtain the matrix below
the determinant of the matrix above is
Next, we check the sign associated with matrix. The matrix sign used to write the co-factor of each elements is given below;
Therefore the sign associated with 5 is -1. Hence the co-factor of 5 is
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