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 · 9 Apr · Shift 1 · Q59
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 · 9 Apr · Shift 1 · Q59

Matrices and Determinants question

2024 · 9 Apr · Shift 1 · Q59

JEE MainMathematicsMatrices and DeterminantsNumerical+4 / −1
Let AAA be a non-singular matrix of order 3. If det⁡(3adj⁡(2adj⁡((det⁡A)A)))=3−13⋅2−10\operatorname{det}(3 \operatorname{adj}(2 \operatorname{adj}((\operatorname{det} A) A)))=3^{-13} \cdot 2^{-10}det(3adj(2adj((detA)A)))=3−13⋅2−10 and det⁡(3adj⁡(2A))=2m⋅3n\operatorname{det}(3\operatorname{adj}(2 \mathrm{A}))=2^{\mathrm{m}} \cdot 3^{\mathrm{n}}det(3adj(2A))=2m⋅3n, then ∣3 m+2n∣|3 \mathrm{~m}+2 \mathrm{n}|∣3 m+2n∣ is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 14

  1. Use determinant and adjoint formulas for a 3×33\times 33×3 matrix

For a matrix MMM of order 333:

det⁡(adj⁡M)=(det⁡M)3−1=(det⁡M)2\det(\operatorname{adj} M)=\big(\det M\big)^{3-1}=(\det M)^2det(adjM)=(detM)3−1=(detM)2

Also, for scalar kkk and 3×33\times 33×3 matrix MMM:

det⁡(kM)=k3det⁡(M)\det(kM)=k^3\det(M)det(kM)=k3det(M)

Let

d=det⁡(A)≠0d=\det(A)\neq 0d=det(A)=0

since AAA is non-singular.


  1. Simplify the inner matrix

We are given

det⁡(3adj⁡(2adj⁡((det⁡A)A)))=3−132−10\det\big(3\operatorname{adj}(2\operatorname{adj}((\det A)A))\big)=3^{-13}2^{-10}det(3adj(2adj((detA)A)))=3−132−10

Start with

B=(det⁡A)A=dAB=(\det A)A=dAB=(detA)A=dA

Then

det⁡(B)=det⁡(dA)=d3det⁡(A)=d4\det(B)=\det(dA)=d^3\det(A)=d^4det(B)=det(dA)=d3det(A)=d4

Now,

det⁡(adj⁡B)=(det⁡B)2=d8\det(\operatorname{adj} B)=(\det B)^2=d^8det(adjB)=(detB)2=d8

So for

C=2adj⁡(B)C=2\operatorname{adj}(B)C=2adj(B)

we get

det⁡(C)=23det⁡(adj⁡B)=23d8\det(C)=2^3\det(\operatorname{adj}B)=2^3d^8det(C)=23det(adjB)=23d8

Now consider

adj⁡(C)\operatorname{adj}(C)adj(C)

Then

det⁡(adj⁡C)=(det⁡C)2=(23d8)2=26d16\det(\operatorname{adj} C)=(\det C)^2=(2^3d^8)^2=2^6d^{16}det(adjC)=(detC)2=(23d8)2=26d16

Finally,

det⁡(3adj⁡C)=33det⁡(adj⁡C)=33⋅26d16\det(3\operatorname{adj}C)=3^3\det(\operatorname{adj}C)=3^3\cdot 2^6 d^{16}det(3adjC)=33det(adjC)=33⋅26d16

This is given equal to

3−132−103^{-13}2^{-10}3−132−10

Hence,

33⋅26d16=3−132−103^3\cdot 2^6 d^{16}=3^{-13}2^{-10}33⋅26d16=3−132−10

So,

d16=3−162−16=(6)−16d^{16}=3^{-16}2^{-16}=(6)^{-16}d16=3−162−16=(6)−16

Therefore,

d=±6−1=±16d=\pm 6^{-1}=\pm \frac{1}{6}d=±6−1=±61​


  1. Now compute det⁡(3adj⁡(2A))\det(3\operatorname{adj}(2A))det(3adj(2A))

Let

E=2AE=2AE=2A

Then

det⁡(E)=23det⁡(A)=8d\det(E)=2^3\det(A)=8ddet(E)=23det(A)=8d

So,

det⁡(adj⁡E)=(det⁡E)2=(8d)2=64d2\det(\operatorname{adj}E)=(\det E)^2=(8d)^2=64d^2det(adjE)=(detE)2=(8d)2=64d2

Therefore,

det⁡(3adj⁡(2A))=33det⁡(adj⁡(2A))=27⋅64d2\det(3\operatorname{adj}(2A))=3^3\det(\operatorname{adj}(2A))=27\cdot 64 d^2det(3adj(2A))=33det(adj(2A))=27⋅64d2

Using d2=(16)2=136d^2=\left(\frac{1}{6}\right)^2=\frac{1}{36}d2=(61​)2=361​,

det⁡(3adj⁡(2A))=27⋅64⋅136\det(3\operatorname{adj}(2A))=27\cdot 64\cdot \frac{1}{36}det(3adj(2A))=27⋅64⋅361​

=2736⋅64=34⋅64=48=\frac{27}{36}\cdot 64=\frac{3}{4}\cdot 64=48=3627​⋅64=43​⋅64=48

Now,

48=24⋅3148=2^4\cdot 3^148=24⋅31

So,

m=4,n=1m=4,\qquad n=1m=4,n=1


  1. Find the required value

∣3m+2n∣=∣3⋅4+2⋅1∣=∣12+2∣=14|3m+2n|=|3\cdot 4+2\cdot 1|=|12+2|=14∣3m+2n∣=∣3⋅4+2⋅1∣=∣12+2∣=14


  1. Comparison with stored answer

Derived answer =14=14=14, which matches the stored correct answer.

PreviousNext

More from Matrices and Determinants

  • 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
  • Consider the matrices : A=[23​−5m​],B=[20m​] and X=[xy​]. Let the set of all m, for which…2024 · Numerical
  • Consider the matrix f(x)=​cosxsinx0​−sinxcosx0​001​​. Given below are two statements : Statement I : f(−x) is the inverse of the matrix f(x)…2024 · MCQ
  • Let A=​211​010​101​​,B=[B1​,B2​,B3​], where B1​,B2​,B3​ are column matrics, and AB1​=​100​​,AB2​=​230​​,AB3​=​321​​…2024 · Numerical
  • The values of α, for which ​112α+3​23​31​3α+1​α+23​α+31​0​​=0, lie in the interval2024 · MCQ
  • Let A be a 2×2 real matrix and I be the identity matrix of order 2. If the roots of the equation ∣A−xI∣=0 be −1 and 3, then the sum of the diagonal elements of the matrix A2 is2024 · Numerical
  •  Let A=​100​0αβ​0βα​​ and ∣2 A∣3=221 where α,β∈Z, Then a value of α is …2024 · MCQ
  • Let A be a square matrix such that AAT=I. Then 21​A[(A+AT)2+(A−AT)2] is equal to2024 · MCQ