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Question: Answered & Verified by Expert
If $A, B$ are two non singular matrices of order $3,|B|=k$, a positive integer, then match the items of list-I with the items of list-II.


The correct match is
$\begin{array}{llll}
A & B & C & D
\end{array}$
MathematicsMatricesTS EAMCETTS EAMCET 2020 (10 Sep Shift 1)
Options:
  • A $\begin{array}{llll}\text { III } & \mathrm{V} & \text{II} & \text { IV }\end{array}$
  • B $\begin{array}{llll}\text { III } & \mathrm{IV} & \text{I} & \text { II }\end{array}$
  • C $\begin{array}{llll}\text { I } & \mathrm{V} & \text{II} & \text { IV }\end{array}$
  • D $\begin{array}{llll}\text { III } & \mathrm{IV} & \text{II} & \text { I }\end{array}$
Solution:
1976 Upvotes Verified Answer
The correct answer is: $\begin{array}{llll}\text { III } & \mathrm{V} & \text{II} & \text { IV }\end{array}$
It is given that the matrices $A$ and $B$ are non-singular of order 3 and $|B|=k$, $a$ positive integer, so
$\begin{aligned}
\left|k^{-1} A^{-1}\right| & =\left(k^{-1}\right)^3\left|A^{-1}\right|=\frac{1}{k^3|A|} \quad\left[\because\left|A^{-1}\right|=\frac{1}{|A|}\right] \\
& =\frac{1}{|B|^3|A|} \\
\because\left|\operatorname{adj}\left(A^{-1}\right)\right| & =\left|A^{-1}\right|^{3-1}=\left|A^{-1}\right|^2=\frac{1}{|A|^2}
\end{aligned}$
$\left[\because|\operatorname{adj}(A)|=|A|^{n-1}\right.$, where $n$ is the order of matrix $A$. ]
Now, since it is given that $B A B^{-1}=I$
$\begin{aligned}
\Rightarrow \quad A B^{-1} & =B^{-1} \\
\therefore \quad B A^k B^{-1} & =B A^{k-1}\left(A B^{-1}\right)=B A^{k-1} B^{-1} \\
& =B A^{k-2}\left(A B^{-1}\right)=B A^{k-2} B^{-1} \\
\therefore \quad B A^k B^{-1} & =B A B^{-1}=I \\
\text { and }, B \frac{\operatorname{adj}(B)}{|B|} & =B B^{-1}=I
\end{aligned}$
Therefore, $B A^k B^{-1}=B \frac{\operatorname{adj}(B)}{|B|}$, if $B A B^{-1}=I$
Now, the $\operatorname{adj}\left(\operatorname{adj}\left(A^{-1}\right)\right)=\left|A^{-1}\right|^{3-2}\left(A^{-1}\right)$
$\left[\because \operatorname{adj}(\operatorname{adj} A)=|A|^{n-2} A\right.$ where $n$ is the order of matrix $A]$
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$=\frac{1}{|A|}\left(A^{-1}\right)$
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