Let for $$A = \left[ {\matrix{ 1 & 2 & 3 \cr \alpha & 3 & 1 \cr 1 & 1 & 2 \cr } } \right],|A| = 2$$. If $\mathrm{|2\,adj\,(2\,adj\,(2A))| = {32^n}}$, then $3n + \alpha$ is equal to
Solution
$$
\begin{aligned}
& A=\left[\begin{array}{lll}
1 & 2 & 3 \\
\alpha & 3 & 1 \\
1 & 1 & 2
\end{array}\right] \\\\
& |A|=2
\end{aligned}
$$
<br/><br/>$$
\begin{aligned}
\Rightarrow&1(6-1)-2(2 \alpha-1)+3(\alpha-3)=2\\\\
\Rightarrow&5-4 \alpha+2+3 \alpha-9=2\\\\
\Rightarrow&-\alpha-4=0\\\\
\Rightarrow&\alpha=-4
\end{aligned}
$$
<br/><br/>Now, $\mathrm{|2\,adj\,(2\,adj\,(2A))| = {32^n}}$
<br/><br/>$\Rightarrow$ $2^3|\operatorname{adj}(2 \operatorname{adj}(2 A))|$ = ${32^n}$
<br/><br/>$$
\begin{aligned}
\Rightarrow & 8|\operatorname{Adj}(2 \operatorname{Adj}(2 \mathrm{~A}))| = {32^n}\\\\
\Rightarrow & 8\left|\operatorname{Adj}\left(2 \times 2^2 \operatorname{Adj}(\mathrm{A})\right)\right| = {32^n} \\\\
\Rightarrow & 8\left|\operatorname{Adj}\left(2^3 \operatorname{AdjA}\right)\right| = {32^n} \\\\
\Rightarrow & 8\left|2^6 \operatorname{Adj}(\operatorname{AdjA})\right| = {32^n} \\\\
\Rightarrow & 2^3\left(2^6\right)^3|\operatorname{Adj}(\operatorname{Adj})| = {32^n} \\\\
\Rightarrow & 2^3 \cdot 2^{18}|\mathrm{~A}|^4 = {32^n} \\\\
\Rightarrow& 2^{21} \cdot 2^4 = {32^n}\\\\
\Rightarrow& 2^{25} = {32^n}\\\\
\Rightarrow& \left(2^5\right)^5 = {32^n}\\\\
\Rightarrow& (32)^5 = {32^n}
\end{aligned}
$$
<br/><br/>$$
\begin{array}{ll}
\therefore n=5 \\\\
\Rightarrow 3 n+\alpha=15-4=11
\end{array}
$$
About this question
Subject: Mathematics · Chapter: Matrices and Determinants · Topic: Types of Matrices and Operations
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