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Matrices and Determinants question

2012 · Shift 1 · Q37
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  5. /2012 · Shift 1 · Q37

Matrices and Determinants question

2012 · Shift 1 · Q37

JEE AdvancedMathematicsMatrices and DeterminantsMCQ+3 / −1
Let P=[aij]P = [{a_{ij}}]P=[aij​] be a 3 ×\times× 3 matrix and let Q=[bij]Q = [{b_{ij}}]Q=[bij​], where bij=2i+jaij{b_{ij}} = {2^{i + j}}{a_{ij}}bij​=2i+jaij​ for 1≤i,j≤31 \le i,j \le 31≤i,j≤3. If the determinant of P is 2, then the determinant of the matrix Q is
  1. A
    210
  2. B
    211
  3. C
    212
  4. D
    213
View written solutionFree

Correct answer: D

  1. We are given P=[aij],Q=[bij],bij=2i+jaij,1≤i,j≤3.P=[a_{ij}], \qquad Q=[b_{ij}], \qquad b_{ij}=2^{i+j}a_{ij}, \quad 1\le i,j\le 3.P=[aij​],Q=[bij​],bij​=2i+jaij​,1≤i,j≤3. Also, det⁡(P)=2.\det(P)=2.det(P)=2.

  2. Write the entries of QQQ in a factorized form: bij=2i+jaij=2i aij 2j.b_{ij}=2^{i+j}a_{ij}=2^i\,a_{ij}\,2^j.bij​=2i+jaij​=2iaij​2j.

    This means QQQ can be written as Q=D1 P D2,Q=D_1\,P\,D_2,Q=D1​PD2​, where

    \qquad D_2=\begin{bmatrix}2^1&0&0\\0&2^2&0\\0&0&2^3\end{bmatrix}.$$
  3. Now use the determinant property: det⁡(Q)=det⁡(D1)det⁡(P)det⁡(D2).\det(Q)=\det(D_1)\det(P)\det(D_2).det(Q)=det(D1​)det(P)det(D2​).

  4. Compute the determinants of the diagonal matrices: det⁡(D1)=21⋅22⋅23=21+2+3=26,\det(D_1)=2^1\cdot 2^2\cdot 2^3=2^{1+2+3}=2^6,det(D1​)=21⋅22⋅23=21+2+3=26, and similarly, det⁡(D2)=26.\det(D_2)=2^6.det(D2​)=26.

  5. Therefore, det⁡(Q)=26⋅2⋅26=212⋅2=213.\det(Q)=2^6\cdot 2 \cdot 2^6=2^{12}\cdot 2=2^{13}.det(Q)=26⋅2⋅26=212⋅2=213.

  6. So the determinant of QQQ is 213.\boxed{2^{13}}.213​.

  7. Comparing with the options:

    • A: 2102^{10}210
    • B: 2112^{11}211
    • C: 2122^{12}212
    • D: 2132^{13}213

    Hence the correct option is D.

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