Sign in
12thPass logo
New chatPYQ LibraryDoubtsRank report
Sign in to see Recents

Your guest activity stays on this device

Sign in to save progress →
Sign in

Matrices and Determinants question

2025 · 3 Apr · Shift 1 · Q45
Guest · filters and generic practice availableBrowsing as a guest · PYQ filters and generic practice are available. Sign in only for personalised features and saved progress.
  1. PYQ Library
  2. /JEE Main
  3. /Mathematics
  4. /Matrices and Determinants
  5. /2025 · 3 Apr · Shift 1 · Q45

Matrices and Determinants question

2025 · 3 Apr · Shift 1 · Q45

JEE MainMathematicsMatrices and DeterminantsMCQ+4 / −1
Let AAA be a matrix of order 3×33 \times 33×3 and ∣A∣=5|A|=5∣A∣=5. If ∣2adj⁡(3Aadj⁡(2A))∣=2α⋅3β⋅5γ,α,β,γ∈N|2 \operatorname{adj}(3 A \operatorname{adj}(2 A))|=2^\alpha \cdot 3^\beta \cdot 5^\gamma, \alpha, \beta, \gamma \in N∣2adj(3Aadj(2A))∣=2α⋅3β⋅5γ,α,β,γ∈N, then α+β+γ\alpha+\beta+\gammaα+β+γ is equal to
  1. A
    26
  2. B
    27
  3. C
    25
  4. D
    28
View written solutionFree

Correct answer: B

  1. Let B=2 adj⁡(3Aadj⁡(2A)).B=2\,\operatorname{adj}(3A\operatorname{adj}(2A)).B=2adj(3Aadj(2A)). We need to find ∣B∣=∣2 adj⁡(3Aadj⁡(2A))∣.|B|=\left|2\,\operatorname{adj}(3A\operatorname{adj}(2A))\right|.∣B∣=∣2adj(3Aadj(2A))∣.

  2. First simplify the inner determinant.

    For a 3×33\times 33×3 matrix:

    • ∣kA∣=k3∣A∣|kA|=k^3|A|∣kA∣=k3∣A∣
    • ∣adj⁡(M)∣=∣M∣n−1|\operatorname{adj}(M)|=|M|^{n-1}∣adj(M)∣=∣M∣n−1 for an n×nn\times nn×n matrix, so for n=3n=3n=3, ∣adj⁡(M)∣=∣M∣2.|\operatorname{adj}(M)|=|M|^2.∣adj(M)∣=∣M∣2.

    Since ∣A∣=5|A|=5∣A∣=5, ∣2A∣=23∣A∣=8⋅5=40.|2A|=2^3|A|=8\cdot 5=40.∣2A∣=23∣A∣=8⋅5=40.

    Hence, ∣adj⁡(2A)∣=∣2A∣2=402=1600.|\operatorname{adj}(2A)|=|2A|^2=40^2=1600.∣adj(2A)∣=∣2A∣2=402=1600.

    Also, ∣3A∣=33∣A∣=27⋅5=135.|3A|=3^3|A|=27\cdot 5=135.∣3A∣=33∣A∣=27⋅5=135.

  3. Now compute ∣3Aadj⁡(2A)∣=∣3A∣⋅∣adj⁡(2A)∣=135⋅1600.|3A\operatorname{adj}(2A)|=|3A|\cdot|\operatorname{adj}(2A)|=135\cdot 1600.∣3Aadj(2A)∣=∣3A∣⋅∣adj(2A)∣=135⋅1600.

    So, ∣3Aadj⁡(2A)∣=216000.|3A\operatorname{adj}(2A)|=216000.∣3Aadj(2A)∣=216000.

    Prime factorize: 216000=216⋅1000=24⋅33⋅53.216000=216\cdot 1000=2^4\cdot 3^3\cdot 5^3.216000=216⋅1000=24⋅33⋅53.

  4. Now use the adjoint determinant formula again.

    Let M=3Aadj⁡(2A).M=3A\operatorname{adj}(2A).M=3Aadj(2A). Then ∣adj⁡(M)∣=∣M∣2=(24⋅33⋅53)2=28⋅36⋅56.|\operatorname{adj}(M)|=|M|^2=(2^4\cdot 3^3\cdot 5^3)^2=2^8\cdot 3^6\cdot 5^6.∣adj(M)∣=∣M∣2=(24⋅33⋅53)2=28⋅36⋅56.

  5. Now multiply by the outer scalar 222.

    Since adj⁡(M)\operatorname{adj}(M)adj(M) is also a 3×33\times 33×3 matrix, ∣2adj⁡(M)∣=23 ∣adj⁡(M)∣.|2\operatorname{adj}(M)|=2^3\,|\operatorname{adj}(M)|.∣2adj(M)∣=23∣adj(M)∣.

    Therefore, ∣2adj⁡(M)∣=23⋅28⋅36⋅56=211⋅36⋅56.|2\operatorname{adj}(M)|=2^3\cdot 2^8\cdot 3^6\cdot 5^6=2^{11}\cdot 3^6\cdot 5^6.∣2adj(M)∣=23⋅28⋅36⋅56=211⋅36⋅56.

  6. Compare with 2α⋅3β⋅5γ.2^\alpha\cdot 3^\beta\cdot 5^\gamma.2α⋅3β⋅5γ. We get α=11,β=6,γ=6.\alpha=11,\quad \beta=6,\quad \gamma=6.α=11,β=6,γ=6.

    Hence, α+β+γ=11+6+6=23.\alpha+\beta+\gamma=11+6+6=23.α+β+γ=11+6+6=23.

  7. So the derived answer is 23.\boxed{23}.23​.

  8. Comparison with stored answer:

    • Stored correct answer: B = 27
    • Derived answer: 23

    Therefore, I do not agree with the stored answer.

  9. Quick verification by an alternative compact method:

    adj⁡(2A)=23−1adj⁡(A)=4adj⁡(A).\operatorname{adj}(2A)=2^{3-1}\operatorname{adj}(A)=4\operatorname{adj}(A).adj(2A)=23−1adj(A)=4adj(A). So, 3Aadj⁡(2A)=3A⋅4adj⁡(A)=12Aadj⁡(A)=12∣A∣I=60I.3A\operatorname{adj}(2A)=3A\cdot 4\operatorname{adj}(A)=12A\operatorname{adj}(A)=12|A|I=60I.3Aadj(2A)=3A⋅4adj(A)=12Aadj(A)=12∣A∣I=60I. Then, adj⁡(60I)=602I\operatorname{adj}(60I)=60^2 Iadj(60I)=602I (for a 3×33\times 33×3 matrix), hence 2adj⁡(60I)=2⋅602I=7200I.2\operatorname{adj}(60I)=2\cdot 60^2 I=7200I.2adj(60I)=2⋅602I=7200I. Therefore, ∣2adj⁡(60I)∣=(7200)3=(25⋅32⋅52)3=2153656.|2\operatorname{adj}(60I)|=(7200)^3=(2^5\cdot 3^2\cdot 5^2)^3=2^{15}3^65^6.∣2adj(60I)∣=(7200)3=(25⋅32⋅52)3=2153656.

    This reveals an important correction: the previous step using only determinant properties missed the fact that the matrix itself simplifies to 60I60I60I, and scaling occurs before determinant evaluation.

    Thus actually, α=15, β=6, γ=6,\alpha=15,\ \beta=6,\ \gamma=6,α=15, β=6, γ=6, so α+β+γ=27.\alpha+\beta+\gamma=27.α+β+γ=27.

  10. Final correct answer: 27\boxed{27}27​ which matches option B.

PreviousNext

More from Matrices and Determinants

  • Let I be the identity matrix of order 3×3 and for the matrix A=​λ47​25−1​362​​,∣A∣=−1. Let B be the inverse of the matrix adj(Aadj(A2))…2025 · Numerical
  • Let A=​cosθ0sinθ​010​−sinθ0cosθ​​. If for some θ∈(0,π),A2=AT, then the sum of the diagonal elements of the…2025 · Numerical
  • Let the matrix A=​110​001​010​​ satisfy An=An−2+A2−I for n⩾3. Then the sum of all the elements of A50 is :2025 · MCQ
  • Let A be a 3×3 matrix such that ∣adj(adj(adjA))∣=81. If S={n∈Z:(∣adj(adjA)∣)2(n−1)2​=∣A∣(3n2−5n−4)}…2025 · MCQ
  • Let the system of equations : ​2x+3y+5z=97x+3y−2z=812x+3y−(4+λ)z=16−μ​ have infinitely many solutions. Then the radius of the circle centred at (λ,μ) and…2025 · MCQ
  • The number of singular matrices of order 2 , whose elements are from the set {2,3,6,9}, is ​.2025 · Numerical
  • Let the system of equations x + 5y - z = 1 4x + 3y - 3z = 7 24x + y + λz = μ λ, μ ∈ ℝ, have infinitely many solutions. Then the number of the solutions of this system, if x, y, z are integers and satisfy 7 ≤ x + y + z ≤ 77, is :2025 · MCQ
  • Let α be a solution of x2+x+1=0, and for some a and b in R,[4​a​b​]​1−1−2​16−1−14​132−8​​=[0​0​0​]…2025 · MCQ