Q.If $A$ is an invertible matrix of order $3 \times 3$, then $|A^{-1}| =$ ________ .

Showing the 12 most recent of 20 on this concept.

1. AP EAPCET 2023Set eng-2023-05-15-AN1 markMCQ
   Q.If $\det(AB) = (\det A)(\det B)$ and $A$ is a non-singular matrix of order $3 \times 3$, then $\det(\text{adj } A) =$ (A) $\det(A)$ (B) $(\det(A))^{-1}$ (C) $(\det(A))^2$ (D) $(\det(A))^3$

   ›Reveal solutionSolution

   The standard identity $\det(\operatorname{adj} A) = (\det A)^{n-1}$ for an $n\times n$ matrix gives $(\det A)^2$ for $n=3$. Answer: (C).

   Concept and Intuition

   The adjugate satisfies $A \cdot \operatorname{adj}A = (\det A) I$. Taking determinants of both sides and using $\det(AB)=\det A \det B$ turns this matrix identity into a scalar one, letting us find $\det(\operatorname{adj}A)$ purely from $\det A$ and the matrix size $n$.

   Step-by-Step Solution

   1. Start from $A(\operatorname{adj}A) = (\det A) I_n$.
   2. Take determinants of both sides: $\det(A)\det(\operatorname{adj}A) = \det((\det A) I_n)$.
   3. For a scalar $k$ multiplying an $n\times n$ identity matrix, $\det(kI_n) = k^n$. Here $k=\det A$, so RHS is $(\det A)^n$.
   4. So $\det(A)\det(\operatorname{adj}A) = (\det A)^n$. …

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2. AP EAPCET 2025Set eng-2025-05-26-AN1 markMCQ
   Q.If B is the inverse of a third order matrix A and $\det B = k$, then $(\mathrm{adj}\,(\mathrm{adj}\, A))^{-1} =$ (A) $kB$ (B) $\dfrac{1}{k}B$ (C) $kB^{-1}$ (D) $B + kI$

   ›Reveal solutionSolution

   Using $\operatorname{adj}(\operatorname{adj} A) = (\det A)^{n-2}A$ for $n=3$, together with $\det A = 1/k$, gives $(\operatorname{adj}(\operatorname{adj} A))^{-1}=kB$.

   Concept and Intuition

   For an $n\times n$ invertible matrix, the double-adjugate identity is $\operatorname{adj}(\operatorname{adj} A) = (\det A)^{n-2}A$. For $n=3$ this simplifies neatly to $(\det A)\,A$ — a single power of the determinant times $A$ itself. Combining this with $B=A^{-1}$ (so $\det B = 1/\det A$) lets everything be expressed back in terms of $B$ and $k$.

   Step-by-Step Solution

   1. $B = A^{-1}$ and $\det B = k$ $\Rightarrow$ $\det A = \dfrac{1}{\det B} = \dfrac1k$.
   2. For a $3\times3$ matrix: $\operatorname{adj}(\operatorname{adj} A) = (\det A)^{3-2}A = (\det A)A$.
   3. Take the inverse of both sides:

   $(\operatorname{adj}(\operatorname{adj} A))^{-1} = \left[(\det A)A\right]^{-1} = \frac{1}{\det A}A^{-1} = \frac{1}{\det A}B$

   4. Substitute $\dfrac{1}{\det A} = k$ (from step 1): …

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3. AP EAPCET 2026Set eng-2026-05-18-AN1 markMCQ
   Q.$A, B$ are $3^{rd}$ order non-singular square matrices and $K$ is a real number. Which of the following is true? (A) $Adj(AB) = (Adj\, B)(Adj\, A)$ and $adj(A^{-1}) \neq (adj\, A)^{-1}$ (B) $Adj(KA) = K\, Adj(A)$ and $|KA| = K^3|A|$ (C) $|B^{-1}AB| = |A|$ and $(A+B)^2 = A^2 + 2AB + B^2$ (D) $(adj\, A)^{-1} = \dfrac{A}{|A|}$ and $(AB)^{-1} = B^{-1}A^{-1}$

   ›Reveal solutionSolution

   The key idea is to test each statement using standard matrix properties for non‑singular matrices. Only option (D) contains two statements that are both true.

   1. Check option (A):

      • The first part, $\operatorname{Adj}(AB) = (\operatorname{Adj} B)(\operatorname{Adj} A)$, is a true property of adjugates for square matrices (order doesn’t matter as long as they are square).
      • The second part claims $\operatorname{Adj}(A^{-1}) \neq (\operatorname{Adj} A)^{-1}$. But we know $\operatorname{Adj}(A^{-1}) = |A^{-1}| A = \frac{1}{|A|} A$, and $(\operatorname{Adj} A)^{-1} = \frac{A}{|A|}$ (since $\operatorname{Adj} A = |A| A^{-1}$). These are equal, so the inequality is false. Hence (A) is not fully true.

   2. Check option (B):

      • $\operatorname{Adj}(KA) = K^{n-1} \operatorname{Adj} A$ for an $n \times n$ matrix. Here $n=3$, so $\operatorname{Adj}(KA) = K^{2} \operatorname{Adj} A$, not $K \operatorname{Adj} A$. So the first part is false.
      • The second part $|KA| = K^3 |A|$ is true (since determinant scales by $K^n$). But because the first part is false, (B) is incorrect.

   3. Check option (C):

      • $|B^{-1}AB| = |B^{-1}| |A| |B| = \frac{1}{|B|} |A| |B| = |A|$ — this is true (similarity transformation preserves determinant).
      • However, $(A+B)^2 = A^2 + AB + BA + B^2$. For matrices, $AB \neq BA$ in general, so $(A+B)^2 = A^2 + 2AB + B^2$ holds only if $A$ and $B$ commute. No such condition is given, so this is false. Hence (C) is not fully true.

   4. Check option (D): …

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4. AP EAPCET 2024Set eng-2024-05-21-FN1 markMCQ
   Q.Let $A$ be a $4 \times 4$ matrix and $P$ be its adjoint matrix. If $|P| = \left|\dfrac{A}{2}\right|$, then $|A^{-1}| =$ (A) $\pm \dfrac{1}{4}$ (B) $\pm 8$ (C) $\pm 2$ (D) $\pm 4$

   ›Reveal solutionSolution

   Using |adj(A)| = |A|^(n−1) for n=4 and |kA| = k^n|A|, the given equation reduces to |A|² = 1/16, so |A| = ±1/4 and |A⁻¹| = ±4.

   Concept and Intuition

   Two standard determinant identities are needed here:

   • For an $n\times n$ matrix $A$, $\det(\text{adj}\,A) = (\det A)^{n-1}$ (this follows from $A\cdot\text{adj}(A) = |A|I$, taking determinants of both sides: $|A|\cdot|\text{adj}\,A| = |A|^n$, so if $|A|\neq0$, $|\text{adj}\,A|=|A|^{n-1}$).
   • For a scalar $k$ and $n\times n$ matrix $A$: $\det(kA) = k^n\det(A)$ (each of the $n$ rows contributes a factor of $k$).

   Step-by-Step Solution

   1. $A$ is $4\times4$, so $n=4$. Given $P=\text{adj}(A)$, we have $|P| = |A|^{4-1} = |A|^3$.
   2. $\left|\dfrac{A}{2}\right| = \left(\dfrac12\right)^4|A| = \dfrac{1}{16}|A|$.
   3. Given $|P| = \left|\dfrac{A}{2}\right|$: $|A|^3 = \dfrac{1}{16}|A|$.
   4. Since $A^{-1}$ is asked for, $A$ must be invertible, i.e. $|A|\neq 0$. Divide both sides by $|A|$ (valid since $|A|\neq0$): $|A|^2 = \dfrac{1}{16}$.
   5. So $|A| = \pm\dfrac{1}{4}$. …

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5. AP EAPCET 2023Set eng-2023-05-18-FN1 markMCQ
   Q.If A and B are non-singular matrices and $\det(AB)=(\det A)(\det B)$, then $\big((\det A)(\det B)\big)B^{-1}A^{-1} =$ (A) $\mathrm{Adj}(BA)$ (B) $\mathrm{Adj}(A)+\mathrm{Adj}(B)$ (C) $\mathrm{Adj}(AB)$ (D) $(\mathrm{Adj}\,B)(\mathrm{Adj}\,A)$

   ›Reveal solutionSolution

   Using the identity det(M)M⁻¹ = Adj(M) with M = AB (noting (AB)⁻¹ = B⁻¹A⁻¹) gives the answer directly as Adj(AB).

   Concept and Intuition

   For any invertible square matrix M, the adjugate satisfies Adj(M) = det(M)·M⁻¹. This is a standard identity coming from M·Adj(M) = det(M)·I.

   Step-by-Step Solution

   1. Recall (AB)⁻¹ = B⁻¹A⁻¹ (reverse order rule for inverses of a product).
   2. The given expression is [(det A)(det B)]·B⁻¹A⁻¹ = det(AB)·B⁻¹A⁻¹ (using the given det(AB)=(det A)(det B)).
   3. Rewrite B⁻¹A⁻¹ as (AB)⁻¹.
   4. So the expression equals det(AB)·(AB)⁻¹. …

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6. AP EAPCET 2025Set eng-2025-05-24-FN1 markMCQ
   Q.If $a$ is the determinant of the adjoint of the matrix $\begin{bmatrix} 1 & 1 & 2 \\ 1 & 2 & 3 \\ 2 & 3 & 3 \end{bmatrix}$ and $b$ is the determinant of the inverse of the matrix $\begin{bmatrix} 1 & 2 & 3 \\ 4 & -3 & -1 \\ 2 & 1 & -4 \end{bmatrix}$ then $\dfrac{b+1}{18b} =$ (A) $a$ (B) $10a$ (C) $2+a$ (D) $2a$

   ›Reveal solutionSolution

   This tests the identities $\det(\operatorname{adj}A)=(\det A)^{n-1}$ and $\det(A^{-1})=1/\det A$, then simplifying an algebraic expression in $b$ to match $a$.

   Concept and Intuition

   For an $n\times n$ matrix, $\det(\operatorname{adj}A) = (\det A)^{n-1}$; for $n=3$ this is $(\det A)^2$, always non-negative. Also, since $A\cdot A^{-1}=I$, taking determinants gives $\det(A^{-1}) = 1/\det A$. Both facts let us compute $a$ and $b$ purely from the determinants of the given matrices, then plug into the target expression.

   Step-by-Step Solution

   1. Compute $\det A$ for $A=\begin{bmatrix}1&1&2\\1&2&3\\2&3&3\end{bmatrix}$: $\det A = 1(2\cdot3-3\cdot3) - 1(1\cdot3-3\cdot2) + 2(1\cdot3-2\cdot2) = 1(-3)-1(-3)+2(-1) = -3+3-2=-2$.
   2. $a = \det(\operatorname{adj}A) = (\det A)^{2} = (-2)^2 = 4$.
   3. Compute $\det B$ for $B=\begin{bmatrix}1&2&3\\4&-3&-1\\2&1&-4\end{bmatrix}$: $\det B = 1((-3)(-4)-(-1)(1)) - 2(4(-4)-(-1)(2)) + 3(4(1)-(-3)(2))$ …

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7. AP EAPCET 2026Set eng-2026-05-15-FN1 markMCQ
   Q.For a $3 \times 3$ non singular matrix $A$, if $Adj(Adj(Adj(Adj(A)))) = |A|^n A$, then $n =$ (A) $3$ (B) $4$ (C) $8$ (D) $5$

   ›Reveal solutionSolution

   The key idea is that repeatedly applying the adjugate to a $3\times3$ matrix scales it by a power of its determinant. Using the property $\operatorname{Adj}(\operatorname{Adj}(A)) = |A|^{n-2} A$ for an $n\times n$ matrix, we find that after four adjugates the exponent is $3^4 = 81$, so $n = 81$ — but the problem’s given form $|A|^n A$ forces us to match exponents, leading to $n = 8$.

   We are given a $3 \times 3$ non-singular matrix $A$ and the equation

   $\operatorname{Adj}(\operatorname{Adj}(\operatorname{Adj}(\operatorname{Adj}(A)))) = |A|^n A.$

   We need to find $n$.

   ---

   1. Recall the fundamental adjugate property

   For any invertible $m \times m$ matrix $M$,

   $\operatorname{Adj}(M) = |M| \cdot M^{-1}.$

   This is the definition: the adjugate is the transpose of the cofactor matrix, and it satisfies $M \cdot \operatorname{Adj}(M) = |M| I$.

   2. Apply it once

   Let $A$ be $3 \times 3$. Then

   $\operatorname{Adj}(A) = |A| \cdot A^{-1}.$

   3. Apply it twice

   Now compute $\operatorname{Adj}(\operatorname{Adj}(A))$.

   Let $B = \operatorname{Adj}(A) = |A| A^{-1}$.

   Then

   $\operatorname{Adj}(B) = |B| \cdot B^{-1}.$

   We need $|B|$:

   $|B| = \bigl| |A| A^{-1} \bigr| = |A|^3 \cdot |A^{-1}| = |A|^3 \cdot \frac{1}{|A|} = |A|^2.$

   Also $B^{-1} = (|A| A^{-1})^{-1} = \frac{1}{|A|} A$.

   Thus

   $\operatorname{Adj}(\operatorname{Adj}(A)) = |A|^2 \cdot \frac{1}{|A|} A = |A| \, A.$

   Tip

   For an $m \times m$ matrix, $\operatorname{Adj}(\operatorname{Adj}(A)) = |A|^{m-2} A$. Here $m=3$, so $|A|^{3-2} = |A|^1$, matching our result.

   4. Apply it three times

   Let $C = \operatorname{Adj}(\operatorname{Adj}(A)) = |A| A$.

   Then

   $\operatorname{Adj}(C) = |C| \cdot C^{-1}.$

   Now $|C| = \bigl| |A| A \bigr| = |A|^3 \cdot |A| = |A|^4$.

   And $C^{-1} = (|A| A)^{-1} = \frac{1}{|A|} A^{-1}$.

   So

   $\operatorname{Adj}(C) = |A|^4 \cdot \frac{1}{|A|} A^{-1} = |A|^3 A^{-1}.$

   5. Apply it four times …

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8. AP EAPCET 2024Set eng-2024-05-22-FN1 markMCQ
   Q.If $P$ and $Q$ are two $3 \times 3$ matrices such that $|PQ| = 1$ and $|P| = 9$, then the determinant of adjoint of the matrix $P \cdot Adj\, 3Q$ is (A) $9^4$ (B) $\dfrac{1}{9^4}$ (C) $9^2$ (D) $\dfrac{1}{9^2}$

   ›Reveal solutionSolution

   Using $|Q|=1/|P|\cdot|PQ|$, the scaling property $Adj(kA)=k^{n-1}AdjA$, and $|Adj\,M|=|M|^{n-1}$ for $3\times3$ matrices gives $9^4$.

   Concept and Intuition

   For an $n\times n$ matrix, $|kA|=k^n|A|$, $Adj(kA)=k^{n-1}Adj\,A$, and $|Adj\,A|=|A|^{n-1}$. Chaining these scaling rules for $n=3$ solves the problem without ever computing $P$ or $Q$ explicitly.

   Step-by-Step Solution

   1. $|PQ|=|P||Q|=1\Rightarrow |Q|=1/9$ (since $|P|=9$).
   2. $Adj(3Q)=3^{3-1}\,Adj\,Q=9\,Adj\,Q$.
   3. Let $M=P\cdot Adj(3Q)=9\,(P\cdot Adj\,Q)$.
   4. $|P\cdot Adj\,Q|=|P|\cdot|Adj\,Q|=|P|\cdot|Q|^{3-1}=9\cdot\left(\dfrac19\right)^2=\dfrac{9}{81}=\dfrac19$.
   5. $|M|=9^3\cdot|P\cdot Adj\,Q|=729\cdot\dfrac19=81=9^2$. …

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9. AP EAPCET 2025Set eng-2025-05-22-AN1 markMCQ
   Q.If $A = \begin{bmatrix} 1 & 2 & 3 \\ 1 & 3 & 5 \\ 2 & 1 & 6 \end{bmatrix}$ and $|\mathrm{adj}(\mathrm{adj}\,A)|(\mathrm{adj}\,A)^{-1} = kA$, then $k =$ (A) 1296 (B) 216 (C) 36 (D) 432

   ›Reveal solutionSolution

   Using the standard adjugate identities for a $3\times3$ matrix, $k=|A|^3$; direct computation gives $|A|=6$, so $k=216$ — option (B).

   Concept and Intuition

   The adjugate (classical adjoint) of an $n\times n$ matrix satisfies two workhorse identities: $|\mathrm{adj}\,A| = |A|^{n-1}$, and (applying that twice) $|\mathrm{adj}(\mathrm{adj}\,A)| = |A|^{(n-1)^2}$. Also, since $A\cdot\mathrm{adj}\,A = |A|\,I$, we get $\mathrm{adj}\,A = |A|A^{-1}$, i.e. $(\mathrm{adj}\,A)^{-1} = \dfrac{A}{|A|}$. Combining these turns the whole expression into a scalar power of $|A|$ times $A$ — exactly the form $kA$ the question wants.

   Step-by-Step Solution

   1. For $n=3$: $|\mathrm{adj}\,A|=|A|^{n-1}=|A|^2$, so $|\mathrm{adj}(\mathrm{adj}\,A)| = |\mathrm{adj}\,A|^{n-1} = (|A|^2)^{2} = |A|^4$.
   2. $(\mathrm{adj}\,A)^{-1} = \dfrac{A}{|A|}$ (from $A\,\mathrm{adj}\,A=|A|I$).
   3. So $|\mathrm{adj}(\mathrm{adj}\,A)|\,(\mathrm{adj}\,A)^{-1} = |A|^4\cdot\dfrac{A}{|A|} = |A|^3 A$. Comparing to $kA$: $k=|A|^3$. …

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10. AP EAPCET 2024Set eng-2024-05-21-AN1 markMCQ
    Q.Assertion (A): If $B$ is a $3 \times 3$ matrix and $|B| = 6$, then $|Adj(B)| = 36$. Reason (R): If $B$ is a square matrix of order $n$, then $|Adj(B)| = |B|^n$ (A) Both (A) and (R) are true and (R) is the correct explanation of (A) (B) Both (A) and (R) are true but (R) is not the correct explanation of (A) (C) (A) is true but (R) is false (D) (A) is false but (R) is true

    ›Reveal solutionSolution

    The correct identity is $|\mathrm{Adj}(B)|=|B|^{n-1}$; this makes Assertion (A)'s numeric value (36) actually correct, but Reason (R) states the wrong general formula ($|B|^n$), so (R) is false even though (A) is true.

    Concept and Intuition

    For an $n\times n$ invertible matrix $B$, the adjugate satisfies $B\cdot\mathrm{Adj}(B) = |B| I$. Taking determinants of both sides: $|B|\cdot|\mathrm{Adj}(B)| = |B|^n$ (since $\det(|B|I) = |B|^n$ for an $n\times n$ identity scaled by $|B|$). Dividing by $|B|$ (nonzero, since $B$ is invertible as $|B|=6\ne0$) gives $|\mathrm{Adj}(B)| = |B|^{n-1}$.

    Step-by-Step Solution

    1. Derive the correct formula: from $B\cdot\mathrm{Adj}(B)=|B|I_n$, take determinants: $|B|\cdot|\mathrm{Adj}(B)| = |B|^n$, so $|\mathrm{Adj}(B)|=|B|^{n-1}$ (for invertible $B$).
    2. Apply to this problem: $n=3$, $|B|=6$. Correct value: $|\mathrm{Adj}(B)| = 6^{3-1}=6^2=36$.
    3. Assertion (A) claims $|\mathrm{Adj}(B)|=36$ — this numeric value is correct (matches the true formula's output), so (A) is true.
    4. Reason (R) states the general rule as $|\mathrm{Adj}(B)|=|B|^n$ — but the actual rule is $|B|^{n-1}$. As a general statement, (R) is false (if you actually used $|B|^n$ here you'd wrongly get $6^3=216\ne36$). …

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11. AP EAPCET 2026Set eng-2026-05-18-FN1 markMCQ
    Q.If $A = \begin{bmatrix} 2 & -3 & 1 \\ -3 & 1 & -2 \\ 1 & -2 & 3 \end{bmatrix}$, then $Adj(A) =$ (A) $\begin{bmatrix} 1 & -7 & 5 \\ -7 & 5 & -1 \\ 5 & -1 & 7 \end{bmatrix}$ (B) $\begin{bmatrix} 1 & 7 & -5 \\ 7 & 5 & 1 \\ -5 & 1 & 7 \end{bmatrix}$ (C) $\begin{bmatrix} -1 & 7 & 5 \\ 7 & 5 & 1 \\ 5 & 1 & -7 \end{bmatrix}$ (D) $\begin{bmatrix} -1 & 7 & -5 \\ 7 & -5 & 1 \\ -5 & 1 & -7 \end{bmatrix}$

    ›Reveal solutionSolution

    $A$ is symmetric, so its adjoint equals its cofactor matrix; computing the cofactors gives $\begin{bmatrix}-1&7&5\\7&5&1\\5&1&-7\end{bmatrix}$.

    Setup. $\operatorname{Adj}(A)$ is the transpose of the cofactor matrix. Here

    $A=\begin{bmatrix}2 & -3 & 1\\-3 & 1 & -2\\1 & -2 & 3\end{bmatrix}$

    is symmetric, so the adjoint is also symmetric.

    Cofactors.

    $C_{11}=\begin{vmatrix}1&-2\\-2&3\end{vmatrix}=3-4=-1,\quad C_{12}=-\begin{vmatrix}-3&-2\\1&3\end{vmatrix}=-(-9+2)=7,\quad C_{13}=\begin{vmatrix}-3&1\\1&-2\end{vmatrix}=6-1=5,$

    $C_{22}=\begin{vmatrix}2&1\\1&3\end{vmatrix}=6-1=5,\quad C_{23}=-\begin{vmatrix}2&-3\\1&-2\end{vmatrix}=-(-4+3)=1,\quad C_{33}=\begin{vmatrix}2&-3\\-3&1\end{vmatrix}=2-9=-7.$ …

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12. AP EAPCET 2025Set eng-2025-05-21-AN1 markMCQ
    Q.If $A=\begin{pmatrix}a & b & c\\ d & e & f\\ l & m & n\end{pmatrix}$ is a matrix such that $|A|>0$ and $\mathrm{Adj}\,A=\begin{pmatrix}0 & 4 & -6\\ 10 & 8 & 0\\ 2 & 4 & -4\end{pmatrix}$, then $\dfrac{cd}{fb}+\dfrac{ln}{em}=$ (A) $2a$ (B) $a+m$ (C) $a+b$ (D) $a$

    ›Reveal solutionSolution

    Reconstructing $A$ from $\mathrm{adj}(A)=A^{-1}\det(A)$ via $A=\mathrm{adj}(\mathrm{adj}\,A)/\det(A)$ gives concrete entries, letting $\frac{cd}{fb}+\frac{log}{em}$ be evaluated directly and matched to $a+m$.

    Concept and Intuition

    Since $A\cdot\mathrm{adj}(A)=\det(A)\cdot I$, once $\mathrm{adj}(A)$ is fully known and $\det(A)$ is pinned down, $A$ itself is uniquely determined: $A=\det(A)\cdot[\mathrm{adj}(A)]^{-1}=\dfrac{\mathrm{adj}(\mathrm{adj}\,A)}{\det(A)}$ (using $\mathrm{adj}(\mathrm{adj}\,A)=\det(A)^{n-2}A$ for $n=3$). This turns an abstract cofactor question into a concrete numeric matrix, from which any algebraic combination of entries can just be computed.

    Step-by-Step Solution

    1. $\det(\mathrm{adj}\,A)=\det(A)^{2}$ for a $3\times3$ matrix. Computing $\det\begin{pmatrix}0&4&-6\\10&8&0\\2&4&-4\end{pmatrix}=16$, so $\det(A)^2=16\Rightarrow\det(A)=\pm4$; given $|A|>0$, $\det(A)=4$.
    2. Compute $\mathrm{adj}(\mathrm{adj}\,A)$ (cofactor-transpose of the given adjugate matrix): $\begin{pmatrix}-32&-8&48\\40&12&-60\\24&8&-40\end{pmatrix}$.
    3. $A=\dfrac{\mathrm{adj}(\mathrm{adj}\,A)}{\det A}=\dfrac14\begin{pmatrix}-32&-8&48\\40&12&-60\\24&8&-40\end{pmatrix}=\begin{pmatrix}-8&-2&12\\10&3&-15\\6&2&-10\end{pmatrix}$.
    4. Read off: $a=-8,\,b=-2,\,c=12,\,d=10,\,e=3,\,f=-15,\,l=6,\,m=2,\,n=-10$. …

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