Let A and B be two square matrices of order 3 such that det(A) = 3 and det(B) = -4. Find the value of det(-6AB

Question

\det(A) = 3

CBSECBSE Class XII Board 2025Subjective· 2mImportance★★★★★

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Concept understanding — Determinant Evaluation Using Identities

Determinant Evaluation Using Identities

Expanding a $4\times4$ or $5\times5$ determinant term by term is painful and error-prone. The smarter route is to transform the determinant into an easy form using properties (the "identities") that change its value in a known, controlled way — then read the answer off a triangular matrix.

The geometric intuition

A determinant measures the signed "volume" of the box spanned by the rows in $n$ -dimensional space. Sliding one row parallel to another doesn't change that volume; swapping two rows flips its sign; scaling a row scales the volume. The algebraic identities are just these facts translated into rules.

The three row (or column) operations

\det \to -\det

The identical rules hold for columns. There is also row-wise linearity: if a row is a sum $R_i = R_i' + R_i''$ , the determinant splits into the sum of two determinants with all other rows fixed.

Watch out

Row-wise linearity is not $\det(A+B)=\det A+\det B$ — that is false. The splitting works one row at a time.

The strategy

Use operation 3 to create zeros in a row or column (value unchanged).
Factor out common factors with operation 2.
Swap rows if needed to reach upper-triangular form (track the sign change).
The determinant is then the product of the diagonal entries.

Worked example

\det\begin{pmatrix}1&2&3\\4&5&6\\7&8&10\end{pmatrix}.

Apply $R_2\to R_2-4R_1$ and $R_3\to R_3-7R_1$ (no change), then $R_3\to R_3-2R_2$ : …

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The determinant of a product is the product of the determinants, and scaling a matrix by a constant $k$ multiplies its determinant by $k^n$ (where $n$ is the order). For $3 \times 3$ matrices, $\det(-6AB) = (-6)^3 \cdot \det(A) \cdot \det(B) = -216 \cdot 3 \cdot (-4) = 2592$ .

The core idea here is how determinants behave under two common operations: multiplication of matrices and scaling of a matrix by a constant. These are not arbitrary rules — they follow from the fundamental property that the determinant measures how volumes (or oriented volumes) scale under a linear transformation.

When you multiply two matrices $A$ and $B$ , the combined transformation first applies $B$ , then $A$ . The total scaling of volume is the product of the individual scalings. That is why $\det(AB) = \det(A) \cdot \det(B)$ .

When you multiply a matrix by a constant $c$ , you are scaling every entry. For an $n \times n$ matrix, this is equivalent to scaling each of the $n$ rows (or columns) by $c$ . Since the determinant is multilinear — linear in each row — scaling all $n$ rows multiplies the determinant by $c^n$ . So $\det(cA) = c^n \det(A)$ .

Here, $n = 3$ , $c = -6$ , and we have a product $AB$ inside the determinant. So we apply both rules in order.

Handle the product first. $\det(-6AB) = \det\big( (-6) \cdot (AB) \big)$ . This is a scalar $(-6)$ times the matrix $AB$ . For a $3 \times 3$ matrix,

\det(cM) = c^3 \det(M).

\det(-6AB) = (-6)^3 \cdot \det(AB).

Compute $(-6)^3$ . $(-6)^3 = -216$ . So we have

\det(-6AB) = -216 \cdot \det(AB).

Use the product rule for determinants. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 59 on this concept.

CBSE 2026Set 65/1/11 markMCQ
$\Delta_1 = \begin{vmatrix} 1 & 0 & 0 \\ 0 & 2 & 0 \\ 0 & 0 & 3 \end{vmatrix}$
›Reveal solutionSolution

The first determinant is diagonal; the second requires one row interchange to reach diagonal form. Each interchange flips the sign; evaluating both determinants gives $\Delta_2 = -2\Delta_1$ . The answer is (B).

Why determinants change under row operations

A determinant measures the signed volume of the parallelepiped spanned by the row vectors. When you swap two rows, you reflect the figure across a hyperplane—the volume stays the same in magnitude but the orientation reverses, flipping the sign.

The diagonal determinant is the easiest to compute: the product of the diagonal entries. The second determinant looks scrambled, but a single row swap will bring it into a form we recognize.

Step-by-step evaluation

1. Compute $\Delta_1$ directly.

The matrix is diagonal:
$\Delta_1 = \begin{vmatrix} 1 & 0 & 0 \\ 0 & 2 & 0 \\ 0 & 0 & 3 \end{vmatrix} = 1 \cdot 2 \cdot 3 = 6.$
2. Recognize the structure of $\Delta_2$ .
$\Delta_2 = \begin{vmatrix} 0 & 2 & 0 \\ 1 & 0 & 0 \\ 0 & 0 & 6 \end{vmatrix}.$
The first two rows are out of order compared to a diagonal form. Swap rows 1 and 2 to bring the $1$ into the top-left position.

3. Apply the row-interchange property.

Swapping rows 1 and 2:
$\Delta_2 = -\begin{vmatrix} 1 & 0 & 0 \\ 0 & 2 & 0 \\ 0 & 0 & 6 \end{vmatrix}.$
The negative sign comes from the single interchange.

4. Evaluate the new diagonal determinant.
$\begin{vmatrix} 1 & 0 & 0 \\ 0 & 2 & 0 \\ 0 & 0 & 6 \end{vmatrix} = 1 \cdot 2 \cdot 6 = 12.$
So $\Delta_2 = -12$ . …
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CBSE 2026Set A1 markMCQ
$\begin{vmatrix} 23 & 12 & 11 \\ 36 & 10 & 26 \\ 63 & 26 & 37 \end{vmatrix} =$
(a) $1$
(b) $-1$
(c) $0$
(d) $2$
›Reveal solutionSolution

The determinant equals $0$ because one column is the sum of the other two.

Inspect the columns of

$\begin{vmatrix} 23 & 12 & 11 \\ 36 & 10 & 26 \\ 63 & 26 & 37 \end{vmatrix}.$

Check: $12+11 = 23$ , $10+26 = 36$ , $26+37 = 63$ .

…

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CBSE 2026Set A1 markMCQ
$\begin{vmatrix} \cos 15^\circ & \sin 15^\circ \\ \sin 75^\circ & \cos 75^\circ \end{vmatrix} =$
(a) $1$
(b) $0$
(c) $-1$
(d) $\frac{1}{2}$
›Reveal solutionSolution

The determinant equals $\cos 90^\circ = 0$ .

Expand:

$\begin{vmatrix} \cos 15^\circ & \sin 15^\circ \\ \sin 75^\circ & \cos 75^\circ \end{vmatrix} = \cos 15^\circ\cos 75^\circ - \sin 15^\circ\sin 75^\circ.$

…

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CBSE 2026Set A1 markMCQ
$\begin{vmatrix} a+ib & c+id \\ -c+id & a-ib \end{vmatrix} =$
(a) $a^2 + b^2 + c^2 + d^2$
(b) $a^2 - b^2 - c^2 - d^2$
(c) $a^2 - b^2 + c^2 + d^2$
(d) $a^2 + b^2 + c^2 - d^2$
›Reveal solutionSolution

Expand the $2\times2$ determinant and simplify the complex products.

$\begin{vmatrix} a+ib & c+id \\ -c+id & a-ib \end{vmatrix} = (a+ib)(a-ib) - (c+id)(-c+id).$

First term: $(a+ib)(a-ib) = a^2 - (ib)^2 = a^2 + b^2$ . …

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CBSE 2026Set ANNUAL1 markMCQ
$\begin{vmatrix}x^2-x+1 & x-1\\ x+1 & x+1\end{vmatrix}$
(a) $x^2-x+2$
(b) $x^3+x^2-2$
(c) $x^3-x^2+2$
(d) $x^3+x^2+4$
›Reveal solutionSolution

Expand the $2\times2$ determinant using $\begin{vmatrix}a&b\\c&d\end{vmatrix}=ad-bc$ .

$\Delta = (x^2-x+1)(x+1) - (x-1)(x+1)$

$(x^2-x+1)(x+1) = x^3+1$ (the middle terms cancel).

$(x-1)(x+1) = x^2-1$ .

…

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CBSE 2026Set ANNUAL1 mark
$\Delta = \begin{vmatrix}1 & 2 & 4\\ -1 & 3 & 0\\ 4 & 1 & 0\end{vmatrix}$
›Reveal solutionSolution

Expand the $3\times3$ determinant along the first row.

$\Delta=\begin{vmatrix}1&2&4\\-1&3&0\\4&1&0\end{vmatrix}$

Expanding along row 1: …

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CBSE 2026Set ANNUAL1 mark
$\Delta = \begin{vmatrix}0 & \sin\alpha & -\cos\alpha\\ -\sin\alpha & 0 & \sin\beta\\ \cos\alpha & -\sin\beta & 0\end{vmatrix}$
›Reveal solutionSolution

The matrix is skew-symmetric (each $a_{ij}=-a_{ji}$ ) and every odd-order skew-symmetric matrix has determinant $0$ .

Check: $a_{12}=\sin\alpha=-a_{21}$ , $a_{13}=-\cos\alpha=-a_{31}$ , $a_{23}=\sin\beta=-a_{32}$ , and all diagonal entries are $0$ — so the matrix is skew-symmetric.

…

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CBSE 2026Set ANNUAL1 markMCQ
$\begin{vmatrix} \cos 30^\circ & \sin 30^\circ \\ \sin 30^\circ & \cos 30^\circ \end{vmatrix} =$
(a) $\dfrac{1}{2}$
(b) $\dfrac{\sqrt{3}}{2}$
(c) 0
(d) None of these
›Reveal solutionSolution

This determinant has the form $\cos^2\theta - \sin^2\theta = \cos 2\theta$ .

$\begin{vmatrix} \cos 30^\circ & \sin 30^\circ \\ \sin 30^\circ & \cos 30^\circ \end{vmatrix} = \cos 30^\circ \cdot \cos 30^\circ - \sin 30^\circ \cdot \sin 30^\circ = \cos^2 30^\circ - \sin^2 30^\circ$

…

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CBSE 2026Set ANNUAL1 mark
$Q.Evaluate the determinant \Delta = \begin{vmatrix}1 & 2 & 4\ -1 & 3 & 0\ 4 & 1 & 0\end{vmatrix}.$
›Reveal solutionSolution

Expand the $3\times3$ determinant along the first row (or any row/column) using cofactors.

Working: Expanding along Row 1:

$\Delta = \begin{vmatrix}1 & 2 & 4\\ -1 & 3 & 0\\ 4 & 1 & 0\end{vmatrix}$ …

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CBSE 2025Set 65/4/11 markMCQ
$\det(M) = m$
›Reveal solutionSolution
The key idea is that $MN = mI$ implies $N = m M^{-1}$ , so $\det(N) = m^3 \det(M^{-1}) = m^3 \cdot \frac{1}{m} = m^2$ . The correct option is (D).

The problem gives us two square matrices $M$ and $N$ of order 3, with $\det(M) = m$ and $MN = mI$ , where $I$ is the $3 \times 3$ identity matrix. We need $\det(N)$ .

The central concept here is the relationship between matrix multiplication and determinants. When two matrices multiply to give a scalar times the identity, that scalar is intimately connected to the determinant of the first matrix. The equation $MN = mI$ is not just a product — it tells us that $N$ is essentially a scaled inverse of $M$ .

Why? Because if $MN = mI$ , then multiplying both sides on the left by $M^{-1}$ (assuming $M$ is invertible) gives $N = m M^{-1}$ . But we must first check: is $M$ invertible? Yes — since $\det(M) = m \neq 0$ (the problem doesn't state $m \neq 0$ explicitly, but if $m = 0$ , then $MN = 0$ , which would make $N$ singular and the answer ambiguous; in standard exam contexts, $m$ is taken as a non-zero scalar, often a real number, and the options suggest $m \neq 0$ ). So $M^{-1}$ exists.

Now, the determinant of a scalar multiple of a matrix: for an $n \times n$ matrix $A$ , $\det(kA) = k^n \det(A)$ . Here $n = 3$ , so $\det(m M^{-1}) = m^3 \det(M^{-1})$ .

And we know $\det(M^{-1}) = \frac{1}{\det(M)} = \frac{1}{m}$ .

Putting it together:

From $MN = mI$ , take determinant on both sides: $\det(MN) = \det(mI)$ .

$\det(MN) = \det(M) \cdot \det(N) = m \cdot \det(N)$ . …
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CBSE 2025Set E1 markMCQ
$\begin{vmatrix} 21 & 11 & 10 \\ 25 & 15 & 10 \\ 64 & 27 & 37 \end{vmatrix} =$
(a) $1190$
(b) $841$
(c) $0$
(d) $1$
›Reveal solutionSolution

A column that is the sum of the other two makes the determinant zero.

Examine the columns of

$\begin{vmatrix} 21 & 11 & 10 \\ 25 & 15 & 10 \\ 64 & 27 & 37 \end{vmatrix}.$

Check $C_2+C_3$ against $C_1$ :

$11+10=21,\quad 15+10=25,\quad 27+37=64.$

…

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CBSE 2025Set E1 markMCQ
$\begin{vmatrix} 10 & 4 \\ 13 & 5 \end{vmatrix} =$
(a) $102$
(b) $2$
(c) $-2$
(d) $-102$
›Reveal solutionSolution

Cross-multiply: $ad-bc=-2$ .

For a $2\times2$ determinant $\begin{vmatrix} a & b \\ c & d \end{vmatrix}=ad-bc$ : …

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Q.Let A and B be two square matrices of order 3 such that det⁡(A)=3\det(A) = 3det(A)=3 and det⁡(B)=−4\det(B) = -4det(B)=−4. Find the value of det⁡(−6AB)\det(-6AB)det(−6AB).

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Q.Let A and B be two square matrices of order 3 such that $\det(A) = 3$ and $\det(B) = -4$ . Find the value of $\det(-6AB)$ .

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