Prove that |A adj A| = |A|n. - Vidyadip

Question

Prove that |A adj A| = |A|ⁿ.

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Answer

Let A = [a_ij] be a square matrix of order n × n.

If C_ij is a cofactor of a_ij in A, then adj A = [C_ij]^T = [C_ij]. Also, it is a matrix of order n × n.

Because A and ahj A are matrices of order n × n, A × (adj A) exists and is of order n × n.

$\Rightarrow\left\{\text{A}\times(\text{adj A})\right\}_{\text{ij}}=\sum\limits_{\text{r}-1}^\text{n}\text{A}_{\text{ir }}(\text{adj A})_\text{rj}$

$=\sum\limits_{\text{n}}^{\text{r}-1}\text{a}_{\text{i r}}\text{C}_{\text{r j}}=\begin{cases}|\text{A}| \text{ if i}=\text{j}\\ 0 \text{ otherwise}\end{cases}$

Thus, each diagonal element of A × (adj A) is |A|. Also, the non-diagonal elements are zero.

$\Rightarrow\ \text{A}\times(\text{adj A})=\begin{bmatrix}|\text{A}| & 0 & 0 & ......... & 0 \\ 0 & |\text{A}| & 0 & .......... & 0 \\ 0 & 0 & |\text{A}| & 0 ....... & 0 \\ . \\ . \\ . \\ 0 & 0 & 0 & .......... & |\text{A}|\end{bmatrix} $

$\Rightarrow\ |\text{A}\times(\text{adj A})|=\begin{bmatrix}|\text{A}| & 0 & 0 & ......... & 0 \\ 0 & |\text{A}| & 0 & .......... & 0 \\ 0 & 0 & |\text{A}| & 0 ....... & 0 \\ . \\ . \\ . \\ 0 & 0 & 0 & .......... & |\text{A}|\end{bmatrix} $

$=|\text{A}|^\text{n}\begin{bmatrix} 1 & 0 & 0 & ......... & 0 \\ 0 & 1 & 0 & .......... & 0 \\ 0 & 0 & 1 & 0 ....... & 0 \\ . \\ . \\ . \\ 0 & 0 & 0 & .......... & 1\end{bmatrix} $

$=|\text{A}|^\text{n}\text{I}_\text{n}=|\text{A}|^\text{n}$

$\Rightarrow\ |\text{A}\times(\text{adj A})|=|\text{A}|^\text{n}$

Hence proved.

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