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Mathematics · 2023 · 5 marks
CBSE 2023 · Region 4 · Set 1 · Q32
Find the inverse of the matrix A = [1, -1, 2; 0, 2, -3; 3, -2, 4]. Using the inverse, A^-1, solve the system of linear equations x-y+2 z=1; 2 y-3…
Find the inverse of the matrix $\displaystyle \mathrm{A}=\left[\begin{array}{rrr}1 & -1 & 2 \\ 0 & 2 & -3 \\ 3 & -2 & 4\end{array}\right]$. Using the inverse, $\displaystyle \mathrm{A}^{-1}$, solve the system of linear equations \[x-y+2 z=1 ; 2 y-3 z=1 ; 3 x-2 y+4 z=3 . \]
Marking-scheme solution
$$\mathrm{A}=\left[\begin{array}{rrr}
1 & -1 & 2 \\
0 & 2 & -3 \\
3 & -2 & 4
\end{array}\right]
$$
$\displaystyle \therefore \mathrm{A}$ is invertible.
$$\begin{aligned}
& \operatorname{adj} \mathrm{A}=\left[\begin{array}{rrr}
2 & 0 & -1 \\
-9 & -2 & 3 \\
-6 & -1 & 2
\end{array}\right] \\
& \Rightarrow \mathrm{A}^{-1}=\frac{1}{|\mathrm{A}|} \operatorname{adj} \mathrm{A}=\left[\begin{array}{rrr}
-2 & 0 & 1 \\
9 & 2 & -3 \\
6 & 1 & -2
\end{array}\right]
\end{aligned}
$$
The given system of equation can be written as $\displaystyle \mathrm{AX}=\mathrm{B}$, when
$$\mathrm{A}=\left[\begin{array}{ccc}
1 & -1 & 2 \\
0 & 2 & -3 \\
3 & -2 & 4
\end{array}\right], X=\left[\begin{array}{l}
\mathrm{x} \\
\mathrm{y} \\
\mathrm{z}
\end{array}\right], \mathrm{B}=\left[\begin{array}{l}
1 \\
1 \\
\end{array}\right]
$$
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CBSE Class 12 Mathematics past-paper question from the 2023 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.