Skip to main content
Back to Matrices & Determinants
JEE Main 2022
Matrices & Determinants
Matrices and Determinants
Hard

Question

Let AA be a non-singular matrix of order 3. If det(3adj(2adj((detA)A)))=313210\operatorname{det}(3 \operatorname{adj}(2 \operatorname{adj}((\operatorname{det} A) A)))=3^{-13} \cdot 2^{-10} and det(3adj(2A))=2m3n\operatorname{det}(3\operatorname{adj}(2 \mathrm{A}))=2^{\mathrm{m}} \cdot 3^{\mathrm{n}}, then 3 m+2n|3 \mathrm{~m}+2 \mathrm{n}| is equal to _________.

Answer: 2

Solution

This problem requires a strong grasp of the properties of determinants and adjoints of matrices. We will systematically break down the given determinant expressions using these properties to solve for the unknown values.


Key Concepts and Formulas Used (for a square matrix XX of order n=3n=3 and a scalar kk):

  1. Determinant of a scalar multiple: det(kX)=kndet(X)=k3det(X)\det(kX) = k^n \det(X) = k^3 \det(X)
  2. Determinant of an adjoint matrix: det(adj(X))=(detX)n1=(detX)2\det(\operatorname{adj}(X)) = (\det X)^{n-1} = (\det X)^2
  3. Adjoint of a scalar multiple: adj(kX)=kn1adj(X)=k2adj(X)\operatorname{adj}(kX) = k^{n-1} \operatorname{adj}(X) = k^2 \operatorname{adj}(X)
  4. Adjoint of an adjoint matrix: adj(adj(X))=(detX)n2X=(detX)X\operatorname{adj}(\operatorname{adj}(X)) = (\det X)^{n-2} X = (\det X) X (since n2=1n-2 = 1)

Let D=detAD = \det A for simplicity. Since AA is a non-singular matrix, D0D \neq 0.


Step-by-Step Solution:

We are given two equations involving determinants and need to find the value of 3m+2n|3m+2n|.

Part 1: Analyzing the First Equation to find detA\det A

The first equation is det(3adj(2adj((detA)A)))=313210\det(3 \operatorname{adj}(2 \operatorname{adj}((\det A) A)))=3^{-13} \cdot 2^{-10}.

Let's simplify the expression from the innermost part outwards:

  1. Innermost term: (detA)A=DA(\det A) A = DA. This is a scalar DD multiplied by matrix AA.
  2. Second innermost determinant (argument of outer adj): adj(DA)\operatorname{adj}(DA) Using property 3, adj(kX)=k2adj(X)\operatorname{adj}(kX) = k^2 \operatorname{adj}(X), with k=Dk=D and X=AX=A: adj(DA)=D2adj(A)\operatorname{adj}(DA) = D^2 \operatorname{adj}(A)
  3. Third innermost determinant (argument of outermost adj): 2adj(DA)2 \operatorname{adj}(DA) Substitute the result from step 2: 2(D2adj(A))=2D2adj(A)2(D^2 \operatorname{adj}(A)) = 2D^2 \operatorname{adj}(A)
  4. Outermost adjoint: adj(2D2adj(A))\operatorname{adj}(2D^2 \operatorname{adj}(A)) Using property 3 again, adj(kX)=k2adj(X)\operatorname{adj}(kX) = k^2 \operatorname{adj}(X), with k=2D2k=2D^2 and X=adj(A)X=\operatorname{adj}(A): adj(2D2adj(A))=(2D2)2adj(adj(A))\operatorname{adj}(2D^2 \operatorname{adj}(A)) = (2D^2)^2 \operatorname{adj}(\operatorname{adj}(A)) =4D4adj(adj(A))= 4D^4 \operatorname{adj}(\operatorname{adj}(A)) Now, using property 4, adj(adj(A))=DA\operatorname{adj}(\operatorname{adj}(A)) = DA: =4D4(DA)=4D5A= 4D^4 (DA) = 4D^5 A
  5. Argument of the outermost determinant: 3adj(2adj(DA))3 \operatorname{adj}(2 \operatorname{adj}(DA)) Substitute the result from step 4: 3(4D5A)=12D5A3(4D^5 A) = 12D^5 A
  6. Final determinant evaluation: det(12D5A)\det(12D^5 A) Using property 1, det(kX)=k3det(X)\det(kX) = k^3 \det(X), with k=12D5k=12D^5 and X=AX=A: det(12D5A)=(12D5)3det(A)\det(12D^5 A) = (12D^5)^3 \det(A) =123(D5)3D=123D15D=123D16= 12^3 (D^5)^3 D = 12^3 D^{15} D = 12^3 D^{16} We know 12=22312 = 2^2 \cdot 3, so 123=(223)3=263312^3 = (2^2 \cdot 3)^3 = 2^6 \cdot 3^3. Thus, the LHS simplifies to 2633D162^6 \cdot 3^3 \cdot D^{16}.

Now, equate this to the given value: 2633D16=3132102^6 \cdot 3^3 \cdot D^{16} = 3^{-13} \cdot 2^{-10} To find D16D^{16}, divide both sides by 26332^6 \cdot 3^3: D16=3132102633D^{16} = \frac{3^{-13} \cdot 2^{-10}}{2^6 \cdot 3^3} D16=21063133D^{16} = 2^{-10-6} \cdot 3^{-13-3} D16=216316D^{16} = 2^{-16} \cdot 3^{-16} D16=(23)16=616D^{16} = (2 \cdot 3)^{-16} = 6^{-16} From this, D=detA=±61D = \det A = \pm 6^{-1}. For the subsequent calculations, D2D^2 will be used, which is unique: D2=(61)2=62D^2 = (6^{-1})^2 = 6^{-2}

Part 2: Analyzing the Second Equation to find mm and nn

The second equation is det(3adj(2A))=2m3n\det(3\operatorname{adj}(2 \mathrm{A}))=2^{\mathrm{m}} \cdot 3^{\mathrm{n}}.

Let's simplify the expression from the innermost part outwards:

  1. Innermost term (argument of adj): 2A2A
  2. Adjoint term: adj(2A)\operatorname{adj}(2A) Using property 3, adj(kX)=k2adj(X)\operatorname{adj}(kX) = k^2 \operatorname{adj}(X), with k=2k=2 and X=AX=A: adj(2A)=22adj(A)=4adj(A)\operatorname{adj}(2A) = 2^2 \operatorname{adj}(A) = 4 \operatorname{adj}(A)
  3. Argument of the outermost determinant: 3adj(2A)3 \operatorname{adj}(2A) Substitute the result from step 2: 3(4adj(A))=12adj(A)3(4 \operatorname{adj}(A)) = 12 \operatorname{adj}(A)
  4. Final determinant evaluation: det(12adj(A))\det(12 \operatorname{adj}(A)) Using property 1, det(kX)=k3det(X)\det(kX) = k^3 \det(X), with k=12k=12 and X=adj(A)X=\operatorname{adj}(A): det(12adj(A))=123det(adj(A))\det(12 \operatorname{adj}(A)) = 12^3 \det(\operatorname{adj}(A)) Using property 2, det(adj(A))=(detA)2=D2\det(\operatorname{adj}(A)) = (\det A)^2 = D^2: =123D2= 12^3 D^2 Substitute 123=263312^3 = 2^6 \cdot 3^3: =2633D2= 2^6 \cdot 3^3 \cdot D^2 Now, substitute the value of D2=62D^2 = 6^{-2} found in Part 1: =2633(62)= 2^6 \cdot 3^3 \cdot (6^{-2}) Since 62=(23)2=22326^{-2} = (2 \cdot 3)^{-2} = 2^{-2} \cdot 3^{-2}: =26332232= 2^6 \cdot 3^3 \cdot 2^{-2} \cdot 3^{-2} Combine the powers of 2 and 3: =262332= 2^{6-2} \cdot 3^{3-2} =2431= 2^4 \cdot 3^1

This expression is given as 2m3n2^m \cdot 3^n. Therefore, by comparing the powers: m=4m=4 n=1n=1

Part 3: Calculating the Final Value

We need to find 3m+2n|3m+2n|. Substitute the values of mm and nn: 3m+2n=3(4)+2(1)|3m+2n| = |3(4) + 2(1)| =12+2= |12 + 2| =14= |14| =14= 14

The final answer is 14\boxed{14}.


Summary and Key Takeaway:

This problem demonstrates the importance of meticulously applying the properties of determinants and adjoints. By systematically simplifying the expressions from the innermost operations outwards, we avoided errors. The key properties for an n×nn \times n matrix are det(kX)=kndet(X)\det(kX) = k^n \det(X) and det(adj(X))=(detX)n1\det(\operatorname{adj}(X)) = (\det X)^{n-1}. Additionally, the property adj(kX)=kn1adj(X)\operatorname{adj}(kX) = k^{n-1} \operatorname{adj}(X) is crucial for simplifying complex adjoint arguments. Careful tracking of powers of scalars is essential to arrive at the correct final answer.

Practice More Matrices & Determinants Questions

View All Questions