Mathematics · Matrices

JEE Main 2024 — 6 April, Shift 2 — Question 20

If A is a square matrix of order 3 such that det⁡(A)=3\operatorname{det}(\mathrm{A})=3 and

det⁡(adj⁡(−4adj⁡(−3adj⁡(3adj⁡((2 A)−1)))))=2m3n\operatorname{det}\left(\operatorname{adj}\left(-4 \operatorname{adj}\left(-3 \operatorname{adj}\left(3 \operatorname{adj}\left((2 \mathrm{~A})^{-1}\right)\right)\right)\right)\right)=2^{\mathrm{m}} 3^{\mathrm{n}}, then m+ 2nm+\ 2 n is equal to

  1. Option A:

    3

  2. Option B:

    2

  3. Option C:

    4

    Correct
  4. Option D:

    6

Answer: C

Step-by-step solution

∣A∣=3|\mathrm{A}|=3 ∣adj⁡(−4adj⁡(−3adj⁡(3adj⁡((2 A)−1))))∣\left|\operatorname{adj}\left(-4 \operatorname{adj}\left(-3 \operatorname{adj}\left(3 \operatorname{adj}\left((2 \mathrm{~A})^{-1}\right)\right)\right)\right)\right| ∣−4adj⁡(−3adj⁡(3adj⁡(2 A)−1)∣2\mid-4 \operatorname{adj}\left(-\left.3 \operatorname{adj}\left(3 \operatorname{adj}(2 \mathrm{~A})^{-1}\right)\right|^{2}\right.

46∣adj⁡(−3adj⁡(3adj⁡(2 A)−1)∣24^{6} \mid \operatorname{adj}\left(-\left.3 \operatorname{adj}\left(3 \operatorname{adj}(2 \mathrm{~A})^{-1}\right)\right|^{2}\right.

212⋅312∣3adj⁡(2 A)−1∣82^{12} \cdot 3^{12}\left|3 \operatorname{adj}(2 \mathrm{~A})^{-1}\right|^{8}

212⋅312⋅324∣adj⁡(2 A)−1∣82^{12} \cdot 3^{12} \cdot 3^{24}\left|\operatorname{adj}(2 \mathrm{~A})^{-1}\right|^{8}

212⋅336∣(2 A)−1∣162^{12} \cdot 3^{36}\left|(2 \mathrm{~A})^{-1}\right|^{16}

212⋅3361∣2 A∣162^{12} \cdot 3^{36} \frac{1}{|2 \mathrm{~A}|^{16}}

212⋅3361248∣ A∣162^{12} \cdot 3^{36} \frac{1}{2^{48}|\mathrm{~A}|^{16}}

212⋅3361248⋅3162^{12} \cdot 3^{36} \frac{1}{2^{48} \cdot 3^{16}} 320236=2−36⋅320\frac{3^{20}}{2^{36}}=2^{-36} \cdot 3^{20}

m=−36n=20\mathrm{m}=-36 \quad \mathrm{n}=20

m+2n=4\mathrm{m}+2 \mathrm{n}=4

Answer key and solution verified before publishing.

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Exam
JEE Main 2024
Subject
Mathematics
Chapter
Matrices
Topic
Adjoint of a Square Matrix