Qus : 5
Jamia Millia Islamia MCA PYQ
2
The system of linear equations is:
$a + 2b + 3c = 7$
$2a + 4b + c = 12$
$3a + 6b + 4c = 20$
1
has a unique solution 2
has no solution 3
has infinite number of solutions 4
has two solutions Go to Discussion
Jamia Millia Islamia MCA Previous Year PYQ
Jamia Millia Islamia MCA JAMIA MILLIA ISLAMIA MCA 2023 PYQ
Solution We can write the augmented matrix as:
$\begin{bmatrix}
1 & 2 & 3 & | & 7 \\
2 & 4 & 1 & | & 12 \\
3 & 6 & 4 & | & 20
\end{bmatrix}$
Perform the following operations:
$R_2 \to R_2 - 2R_1$ and $R_3 \to R_3 - 3R_1$
$\Rightarrow
\begin{bmatrix}
1 & 2 & 3 & | & 7 \\
0 & 0 & -5 & | & -2 \\
0 & 0 & -5 & | & -1
\end{bmatrix}$
Now subtract $R_3 - R_2$:
$\Rightarrow
\begin{bmatrix}
0 & 0 & 0 & | & 1
\end{bmatrix}$
This represents an inconsistent equation $0 = 1$.
Hence, the system **has no solution.**
Qus : 6
Jamia Millia Islamia MCA PYQ
3
$Q30.$ If the rank of the matrix
\[
\begin{bmatrix}
a & 0 & 0 \\
0 & b & 0 \\
0 & 0 & c
\end{bmatrix}
\]
is $2$, then find the correct condition.
1
$abc\neq 0$ 2
$a\neq 0, bc=0$ 3
$ab\neq 0, c=0$ 4
$a\neq 0, b\neq 0, c\neq 0$ Go to Discussion
Jamia Millia Islamia MCA Previous Year PYQ
Jamia Millia Islamia MCA JAMIA MILLIA ISLAMIA MCA 2023 PYQ
Solution For a diagonal matrix, the rank equals the number of non-zero diagonal elements.
If the rank is $2$, exactly two of $a, b, c$ must be non-zero and one must be zero.
Thus, the possible condition is
$ab \neq 0,\; c = 0$.
Qus : 14
Jamia Millia Islamia MCA PYQ
1
If $A = \begin{bmatrix} 3 & -9 \\ -12 & 6 \end{bmatrix}$,
then $\operatorname{adj}(3A + 12A^2)$ is equal to:
1
$\begin{bmatrix} 72 & -63 \\ -84 & 51 \end{bmatrix}$
2
$\begin{bmatrix} 63 & -72 \\ -84 & 51 \end{bmatrix}$ 3
$\begin{bmatrix} 81 & -84 \\ -63 & 72 \end{bmatrix}$ 4
$\begin{bmatrix} 72 & -84 \\ -84 & 51 \end{bmatrix}$ Go to Discussion
Jamia Millia Islamia MCA Previous Year PYQ
Jamia Millia Islamia MCA JAMIA MILLIA ISLAMIA MCA 2019 PYQ
Solution $A^2 = \begin{bmatrix} 3 & -9 \\ -12 & 6 \end{bmatrix}^2 =
\begin{bmatrix} 3^2 + (-9)(-12) & 3(-9) + (-9)(6) \\
(-12)(3) + 6(-12) & (-12)(-9) + 6^2 \end{bmatrix}
= \begin{bmatrix} 135 & -81 \\ -108 & 126 \end{bmatrix}$
Then $3A + 12A^2 = 3\begin{bmatrix} 3 & -9 \\ -12 & 6 \end{bmatrix}
+ 12\begin{bmatrix} 135 & -81 \\ -108 & 126 \end{bmatrix}
= \begin{bmatrix} 72 & -63 \\ -84 & 51 \end{bmatrix}$
Hence, $\operatorname{adj}(3A + 12A^2)$ = same matrix (since 2×2 case).
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