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Matrices and Determinants question

2024 · 5 Apr · Shift 1 · Q36
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  5. /2024 · 5 Apr · Shift 1 · Q36

Matrices and Determinants question

2024 · 5 Apr · Shift 1 · Q36

JEE MainMathematicsMatrices and DeterminantsMCQ+4 / −1
Let A and B be two square matrices of order 3 such that ∣A∣=3\mathrm{|A|=3}∣A∣=3 and ∣B∣=2\mathrm{|B|=2}∣B∣=2. Then ∣ATA(adj⁡(2 A))−1(adj⁡(4 B))(adj⁡(AB))−1AAT∣|\mathrm{A}^{\mathrm{T}} \mathrm{A}(\operatorname{adj}(2 \mathrm{~A}))^{-1}(\operatorname{adj}(4 \mathrm{~B}))(\operatorname{adj}(\mathrm{AB}))^{-1} \mathrm{AA}^{\mathrm{T}}|∣ATA(adj(2 A))−1(adj(4 B))(adj(AB))−1AAT∣ is equal to :
  1. A
    32
  2. B
    81
  3. C
    64
  4. D
    108
View written solutionFree

Correct answer: C

  1. We need to find ∣ATA (adj⁡(2A))−1(adj⁡(4B))(adj⁡(AB))−1AAT∣\left|A^T A\,(\operatorname{adj}(2A))^{-1}(\operatorname{adj}(4B))(\operatorname{adj}(AB))^{-1}AA^T\right|​ATA(adj(2A))−1(adj(4B))(adj(AB))−1AAT​ for 3×33\times 33×3 matrices A,BA,BA,B with ∣A∣=3,∣B∣=2.|A|=3,\qquad |B|=2.∣A∣=3,∣B∣=2.

  2. Use the determinant property for a product: ∣PQRS⋯∣=∣P∣ ∣Q∣ ∣R∣ ∣S∣⋯|PQRS\cdots|=|P|\,|Q|\,|R|\,|S|\cdots∣PQRS⋯∣=∣P∣∣Q∣∣R∣∣S∣⋯ So, \begin{align*} &\left|A^T A,(\operatorname{adj}(2A))^{-1}(\operatorname{adj}(4B))(\operatorname{adj}(AB))^{-1}AA^T\right| \ &=|A^TA|\cdot |(\operatorname{adj}(2A))^{-1}|\cdot |\operatorname{adj}(4B)|\cdot |(\operatorname{adj}(AB))^{-1}|\cdot |AA^T|. \end{align*}

  3. Compute the easy determinants first.

For any square matrix MMM, ∣MT∣=∣M∣.|M^T|=|M|.∣MT∣=∣M∣. Hence, ∣ATA∣=∣AT∣∣A∣=∣A∣2=32=9,|A^TA|=|A^T||A|=|A|^2=3^2=9,∣ATA∣=∣AT∣∣A∣=∣A∣2=32=9, and similarly, ∣AAT∣=∣A∣∣AT∣=∣A∣2=9.|AA^T|=|A||A^T|=|A|^2=9.∣AAT∣=∣A∣∣AT∣=∣A∣2=9.

  1. Use the adjoint determinant formula. For an n×nn\times nn×n matrix MMM, ∣adj⁡(M)∣=∣M∣n−1.|\operatorname{adj}(M)|=|M|^{n-1}.∣adj(M)∣=∣M∣n−1. Here n=3n=3n=3, so ∣adj⁡(M)∣=∣M∣2.|\operatorname{adj}(M)|=|M|^2.∣adj(M)∣=∣M∣2. Therefore, ∣(adj⁡(M))−1∣=1∣adj⁡(M)∣=1∣M∣2.|(\operatorname{adj}(M))^{-1}|=\frac{1}{|\operatorname{adj}(M)|}=\frac{1}{|M|^2}.∣(adj(M))−1∣=∣adj(M)∣1​=∣M∣21​.

  2. Compute each remaining factor.

(i) ∣(adj⁡(2A))−1∣|(\operatorname{adj}(2A))^{-1}|∣(adj(2A))−1∣

Since AAA is 3×33\times 33×3, ∣2A∣=23∣A∣=8⋅3=24.|2A|=2^3|A|=8\cdot 3=24.∣2A∣=23∣A∣=8⋅3=24. Thus, ∣adj⁡(2A)∣=∣2A∣2=242,|\operatorname{adj}(2A)|=|2A|^2=24^2,∣adj(2A)∣=∣2A∣2=242, so ∣(adj⁡(2A))−1∣=1242.|(\operatorname{adj}(2A))^{-1}|=\frac{1}{24^2}.∣(adj(2A))−1∣=2421​.

(ii) ∣adj⁡(4B)∣|\operatorname{adj}(4B)|∣adj(4B)∣

∣4B∣=43∣B∣=64⋅2=128.|4B|=4^3|B|=64\cdot 2=128.∣4B∣=43∣B∣=64⋅2=128. Hence, ∣adj⁡(4B)∣=∣4B∣2=1282.|\operatorname{adj}(4B)|=|4B|^2=128^2.∣adj(4B)∣=∣4B∣2=1282.

(iii) ∣(adj⁡(AB))−1∣|(\operatorname{adj}(AB))^{-1}|∣(adj(AB))−1∣

First, ∣AB∣=∣A∣∣B∣=3⋅2=6.|AB|=|A||B|=3\cdot 2=6.∣AB∣=∣A∣∣B∣=3⋅2=6. So, ∣adj⁡(AB)∣=∣AB∣2=62=36,|\operatorname{adj}(AB)|=|AB|^2=6^2=36,∣adj(AB)∣=∣AB∣2=62=36, and therefore, ∣(adj⁡(AB))−1∣=136.|(\operatorname{adj}(AB))^{-1}|=\frac{1}{36}.∣(adj(AB))−1∣=361​.

  1. Multiply all factors: \begin{align*} \text{Required determinant} &=9\cdot \frac{1}{24^2}\cdot 128^2\cdot \frac{1}{36}\cdot 9. \end{align*}

Now simplify: 9⋅9=81,9\cdot 9=81,9⋅9=81, so =81⋅1282242⋅36.=\frac{81\cdot 128^2}{24^2\cdot 36}.=242⋅3681⋅1282​.

Since 1282=16384,242=576,128^2=16384,\qquad 24^2=576,1282=16384,242=576, we get =81⋅16384576⋅36.=\frac{81\cdot 16384}{576\cdot 36}.=576⋅3681⋅16384​.

But a cleaner simplification is: 1282242=(12824)2=(163)2=2569.\frac{128^2}{24^2}=\left(\frac{128}{24}\right)^2=\left(\frac{16}{3}\right)^2=\frac{256}{9}.2421282​=(24128​)2=(316​)2=9256​. Thus, \begin{align*} \text{Required determinant} &=9\cdot \frac{256}{9}\cdot \frac{1}{36}\cdot 9 \ &=256\cdot \frac{9}{36} \ &=256\cdot \frac{1}{4} \ &=64. \end{align*}

  1. Therefore the correct option is 64.\boxed{64}.64​. So, Option C is correct.
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