Answer
The minors are:
$M_{11}=-9$
$M_{12}=7$
$M_{13}=5$
$M_{21}=-7$
$M_{22}=1$
$M_{23}=3$
$M_{31}=-2$
$M_{32}=-2$
$M_{33}=2$
The cofactors of the matrix A are:
$C_{11}=-9$
$C_{12}=-7$
$C_{13}=5$
$C_{21}=7$
$C_{22}=1$
$C_{23}=-3$
$C_{31}=-2$
$C_{32}=2$
$C_{33}=0$
Work Step by Step
$A=\begin{bmatrix}
1 & -1 & 2 \\
3 & 1 & 4 \\
2 & 1 & 5
\end{bmatrix}$
The minors are:
$M_{11}=-9$
$M_{12}=7$
$M_{13}=5$
$M_{21}=-7$
$M_{22}=1$
$M_{23}=3$
$M_{31}=-2$
$M_{32}=-2$
$M_{33}=2$
The cofactors of the matrix A are:
$C_{11}=(-1)^{1+1}.4=-9$
$C_{12}=(-1)^{1+2}.2=-7$
$C_{13}=(-1)^{1+3}.2=5$
$C_{21}=(-1)^{2+1}.(-3)=7$
$C_{22}=(-1)^{2+2}.1=1$
$C_{23}=(-1)^{2+3}.1=-3$
$C_{31}=(-1)^{3+1}.1=-2$
$C_{32}=(-1)^{3+2}.1=2$
$C_{33}=(-1)^{3+3}.1=0$
