If $$\mathrm{A}=\frac{1}{5 ! 6 ! 7 !}\left[\begin{array}{ccc}5 ! & 6 ! & 7 ! \\ 6 ! & 7 ! & 8 ! \\ 7 ! & 8 ! & 9 !\end{array}\right]$$, then $|\operatorname{adj}(\operatorname{adj}(2 \mathrm{~A}))|$ is equal to :
Solution
Given that
<br/><br/>$$
\begin{aligned}
& A=\frac{1}{5 ! 6 ! 7 !}\left[\begin{array}{lll}
5 ! & 6 ! & 7 ! \\
6 ! & 7 ! & 8 ! \\
7 ! & 8 ! & 9 !
\end{array}\right] \\\\
& \Rightarrow|A|=\frac{1}{5 ! 6 ! 7 !}\left|\begin{array}{lll}
5 ! & 6 ! & 7 ! \\
6 ! & 7 ! & 8 ! \\
7 ! & 8 ! & 9 !
\end{array}\right| \\\\
& \Rightarrow|A|=\frac{1}{5 ! 6 ! 7 !} \times 5 ! 6 ! 7 !\left|\begin{array}{lll}
1 & 6 & 7 \times 6 \\
1 & 7 & 8 \times 7 \\
1 & 8 & 9 \times 8
\end{array}\right|
\end{aligned}
$$
<br/><br/>$$
\begin{aligned}
& R_3 \rightarrow R_3-R_2 \text { and } R_2 \rightarrow R_2-R_1 \\\\
& |A|=\left|\begin{array}{lll}
1 & 6 & 42 \\
0 & 1 & 14 \\
0 & 1 & 16
\end{array}\right|=2 \\\\
& |\operatorname{adj}(\operatorname{adj}(2 A))|=|2 A|^{(n-1)^2} \\\\
& =|2 A|^4=\left(2^3|A|\right)^4=2^{12}|A|^4=2^{16}
\end{aligned}
$$
About this question
Subject: Mathematics · Chapter: Matrices and Determinants · Topic: Types of Matrices and Operations
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