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

2024 · 6 Apr · Shift 2 · Q35
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. /2024 · 6 Apr · Shift 2 · Q35

Matrices and Determinants question

2024 · 6 Apr · Shift 2 · Q35

JEE MainMathematicsMatrices and DeterminantsMCQ+4 / −1
If AAA is a square matrix of order 3 such that det⁡(A)=3\operatorname{det}(A)=3det(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}}det(adj(−4adj(−3adj(3adj((2 A)−1)))))=2m3n, then m+2n\mathrm{m}+2 \mathrm{n}m+2n is equal to :
  1. A
    2
  2. B
    4
  3. C
    3
  4. D
    6
View written solutionFree

Correct answer: B

  1. Useful determinant facts for a 3×33\times 33×3 matrix

For a matrix of order 333:

  • det⁡(adj⁡(M))=(det⁡M)3−1=(det⁡M)2\det(\operatorname{adj}(M)) = (\det M)^{3-1} = (\det M)^2det(adj(M))=(detM)3−1=(detM)2
  • det⁡(kM)=k3det⁡(M)\det(kM)=k^3\det(M)det(kM)=k3det(M)
  • det⁡(M−1)=1det⁡(M)\det(M^{-1})=\frac{1}{\det(M)}det(M−1)=det(M)1​

Given: det⁡(A)=3\det(A)=3det(A)=3

We need to evaluate det⁡(adj⁡(−4 adj⁡(−3 adj⁡(3 adj⁡((2A)−1)))))=2m3n\det\left(\operatorname{adj}\left(-4 \, \operatorname{adj}\left(-3 \, \operatorname{adj}\left(3 \, \operatorname{adj}\left((2A)^{-1}\right)\right)\right)\right)\right)=2^m3^ndet(adj(−4adj(−3adj(3adj((2A)−1)))))=2m3n


  1. Start from the innermost matrix

Let M1=(2A)−1M_1=(2A)^{-1}M1​=(2A)−1 Then det⁡(2A)=23det⁡(A)=8⋅3=24\det(2A)=2^3\det(A)=8\cdot 3=24det(2A)=23det(A)=8⋅3=24 So det⁡(M1)=det⁡((2A)−1)=124=2−33−1\det(M_1)=\det\big((2A)^{-1}\big)=\frac{1}{24}=2^{-3}3^{-1}det(M1​)=det((2A)−1)=241​=2−33−1


  1. First adjugate

Let M2=adj⁡(M1)M_2=\operatorname{adj}(M_1)M2​=adj(M1​) Then det⁡(M2)=det⁡(adj⁡(M1))=(det⁡M1)2=(124)2=2−63−2\det(M_2)=\det(\operatorname{adj}(M_1))=(\det M_1)^2=\left(\frac{1}{24}\right)^2=2^{-6}3^{-2}det(M2​)=det(adj(M1​))=(detM1​)2=(241​)2=2−63−2

Now multiply by 333: M3=3M2M_3=3M_2M3​=3M2​ Hence det⁡(M3)=33det⁡(M2)=27⋅2−63−2=2−631\det(M_3)=3^3\det(M_2)=27\cdot 2^{-6}3^{-2}=2^{-6}3^1det(M3​)=33det(M2​)=27⋅2−63−2=2−631


  1. Second adjugate

Let M4=adj⁡(M3)M_4=\operatorname{adj}(M_3)M4​=adj(M3​) Then det⁡(M4)=(det⁡M3)2=(2−631)2=2−1232\det(M_4)=(\det M_3)^2=(2^{-6}3^1)^2=2^{-12}3^2det(M4​)=(detM3​)2=(2−631)2=2−1232

Now multiply by −3-3−3: M5=−3M4M_5=-3M_4M5​=−3M4​ Since order is 333, det⁡(M5)=(−3)3det⁡(M4)=−27⋅2−1232=−2−1235\det(M_5)=(-3)^3\det(M_4)=-27\cdot 2^{-12}3^2=-2^{-12}3^5det(M5​)=(−3)3det(M4​)=−27⋅2−1232=−2−1235


  1. Third adjugate

Let M6=adj⁡(M5)M_6=\operatorname{adj}(M_5)M6​=adj(M5​) Then det⁡(M6)=(det⁡M5)2=(−2−1235)2=2−24310\det(M_6)=(\det M_5)^2=(-2^{-12}3^5)^2=2^{-24}3^{10}det(M6​)=(detM5​)2=(−2−1235)2=2−24310

Now multiply by −4-4−4: M7=−4M6M_7=-4M_6M7​=−4M6​ So det⁡(M7)=(−4)3det⁡(M6)=−64⋅2−24310=−26⋅2−24310=−2−18310\det(M_7)=(-4)^3\det(M_6)=-64\cdot 2^{-24}3^{10}=-2^6\cdot 2^{-24}3^{10}=-2^{-18}3^{10}det(M7​)=(−4)3det(M6​)=−64⋅2−24310=−26⋅2−24310=−2−18310


  1. Final adjugate

Required matrix is M8=adj⁡(M7)M_8=\operatorname{adj}(M_7)M8​=adj(M7​) Therefore det⁡(M8)=(det⁡M7)2=(−2−18310)2=2−36320\det(M_8)=(\det M_7)^2=(-2^{-18}3^{10})^2=2^{-36}3^{20}det(M8​)=(detM7​)2=(−2−18310)2=2−36320

So, 2m3n=2−363202^m3^n=2^{-36}3^{20}2m3n=2−36320 which gives m=−36,n=20m=-36,\quad n=20m=−36,n=20

Hence m+2n=−36+40=4m+2n=-36+40=4m+2n=−36+40=4


  1. Check with options

The value is 4\boxed{4}4​ So the correct option is B.

PreviousNext

More from Matrices and Determinants

  • If the system of equations ​2x+7y+λz=33x+2y+5z=4x+μy+32z=−1​ has infinitely many solutions, then (λ−μ) is equal to ​ :2024 · Numerical
  • Let A=​210​a35​01b​​. If A3=4A2−A−21I, where I is the identity matrix of order 3×3, then 2a+3b is equal to2024 · MCQ
  • Let A=[21​−11​]. If the sum of the diagonal elements of A13 is 3n, then n is equal to ​.2024 · Numerical
  • If αeqa,βeqb,γeqc and ​αaa​bβb​ccγ​​=0…2024 · MCQ
  • If the system of equations x+4y−z=λ,7x+9y+μz=−3,5x+y+2z=−1 has infinitely many solutions, then (2μ+3λ) is equal to :2024 · MCQ
  • Let λ,μ∈R. If the system of equations ​3x+5y+λz=37x+11y−9z=297x+155y−189z=μ​ has infinitely many solutions, then μ+2λ is equal to…2024 · MCQ
  • Let A be a non-singular matrix of order 3. If det(3adj(2adj((detA)A)))=3−13⋅2−10 and det(3adj(2A))=2m⋅3n…2024 · Numerical
  • Let B=[11​35​] and A be a 2×2 matrix such that AB−1=A−1. If BCB−1=A and C4+αC2+βI=O, then 2β−α is equal to2024 · MCQ