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Matrices and Determinants question

2012 · Shift 2 · Q39
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  5. /2012 · Shift 2 · Q39

Matrices and Determinants question

2012 · Shift 2 · Q39

JEE AdvancedMathematicsMatrices and DeterminantsMultiple correct+4 / −1
If the ad joint of a 3 ×\times× 3 matrix P is [144217113]\left[ {\begin{matrix} 1 & 4 & 4 \\ 2 & 1 & 7 \\ 1 & 1 & 3 \\ \end{matrix} } \right]​121​411​473​​, then the possible value(s) of the determinant of P is(are)
  1. A
    −-− 2
  2. B
    −-− 1
  3. C
    1
  4. D
    2
View written solutionFree

Correct answer: A, D

Step-by-step Solution:

  1. Recall the property of adjoint matrices: For any n×nn \times nn×n matrix AAA, the determinant of its adjoint is related to the determinant of the matrix itself by the formula: det⁡(adj(A))=(det⁡(A))n−1\det(\text{adj}(A)) = (\det(A))^{n-1}det(adj(A))=(det(A))n−1

  2. Apply the property to the given matrix P: In this problem, P is a 3×33 \times 33×3 matrix, so n=3n=3n=3. The formula becomes: det⁡(adj(P))=(det⁡(P))3−1=(det⁡(P))2\det(\text{adj}(P)) = (\det(P))^{3-1} = (\det(P))^2det(adj(P))=(det(P))3−1=(det(P))2

  3. Calculate the determinant of the given adjoint matrix: We are given the adjoint of P as: adj(P)=[144217113]\text{adj}(P) = \left[ {\begin{matrix} 1 & 4 & 4 \\ 2 & 1 & 7 \\ 1 & 1 & 3 \\ \end{matrix} } \right]adj(P)=​121​411​473​​ Let's calculate its determinant: det⁡(adj(P))=1∣1713∣−4∣2713∣+4∣2111∣\det(\text{adj}(P)) = 1 \begin{vmatrix} 1 & 7 \\ 1 & 3 \end{vmatrix} - 4 \begin{vmatrix} 2 & 7 \\ 1 & 3 \end{vmatrix} + 4 \begin{vmatrix} 2 & 1 \\ 1 & 1 \end{vmatrix}det(adj(P))=1​11​73​​−4​21​73​​+4​21​11​​ det⁡(adj(P))=1((1)(3)−(7)(1))−4((2)(3)−(7)(1))+4((2)(1)−(1)(1))\det(\text{adj}(P)) = 1((1)(3) - (7)(1)) - 4((2)(3) - (7)(1)) + 4((2)(1) - (1)(1))det(adj(P))=1((1)(3)−(7)(1))−4((2)(3)−(7)(1))+4((2)(1)−(1)(1)) det⁡(adj(P))=1(3−7)−4(6−7)+4(2−1)\det(\text{adj}(P)) = 1(3 - 7) - 4(6 - 7) + 4(2 - 1)det(adj(P))=1(3−7)−4(6−7)+4(2−1) det⁡(adj(P))=1(−4)−4(−1)+4(1)\det(\text{adj}(P)) = 1(-4) - 4(-1) + 4(1)det(adj(P))=1(−4)−4(−1)+4(1) det⁡(adj(P))=−4+4+4\det(\text{adj}(P)) = -4 + 4 + 4det(adj(P))=−4+4+4 det⁡(adj(P))=4\det(\text{adj}(P)) = 4det(adj(P))=4

  4. Solve for the determinant of P: Substitute the value of det⁡(adj(P))\det(\text{adj}(P))det(adj(P)) into the equation from Step 2: (det⁡(P))2=4(\det(P))^2 = 4(det(P))2=4 Taking the square root of both sides, we get: det⁡(P)=±4\det(P) = \pm \sqrt{4}det(P)=±4​ det⁡(P)=±2\det(P) = \pm 2det(P)=±2

  5. Conclusion: The possible values for the determinant of P are 2 and -2.

Evaluating the Options:

  • A: -2 - This is a possible value.
  • B: -1 - This is not a possible value.
  • C: 1 - This is not a possible value.
  • D: 2 - This is a possible value.

Therefore, the correct options are A and D.

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