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Matrices and Determinants question

2024 · 9 Apr · Shift 2 · Q44
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Matrices and Determinants question

2024 · 9 Apr · Shift 2 · Q44

JEE MainMathematicsMatrices and DeterminantsMCQ+4 / −1
Let B=[1315]B=\left[\begin{array}{ll}1 & 3 \\ 1 & 5\end{array}\right]B=[11​35​] and AAA be a 2×22 \times 22×2 matrix such that AB−1=A−1A B^{-1}=A^{-1}AB−1=A−1. If BCB−1=AB C B^{-1}=ABCB−1=A and C4+αC2+βI=OC^4+\alpha C^2+\beta I=OC4+αC2+βI=O, then 2β−α2 \beta-\alpha2β−α is equal to
  1. A
    16
  2. B
    10
  3. C
    8
  4. D
    2
View written solutionFree

Correct answer: B

  1. Given relation

We have B=[1315]B=\begin{bmatrix}1&3\\1&5\end{bmatrix}B=[11​35​] and AB−1=A−1.AB^{-1}=A^{-1}.AB−1=A−1.

We want to use this to find a polynomial satisfied by AAA, then by CCC.


  1. Derive an equation for AAA

From AB−1=A−1,AB^{-1}=A^{-1},AB−1=A−1, multiply on the right by BBB: A=A−1B.A=A^{-1}B.A=A−1B. Now multiply on the left by AAA: A2=B.A^2=B.A2=B.

So AAA is a square root of BBB.


  1. Use similarity of AAA and CCC

Given BCB−1=A,BCB^{-1}=A,BCB−1=A, this means AAA and CCC are similar matrices. Hence they satisfy the same characteristic polynomial, and any polynomial equation satisfied by AAA is also satisfied by CCC.

Since A2=BA^2=BA2=B, let us first find a polynomial satisfied by BBB.


  1. Find the characteristic polynomial of BBB

For B=[1315],B=\begin{bmatrix}1&3\\1&5\end{bmatrix},B=[11​35​], its trace and determinant are tr⁡(B)=1+5=6,\operatorname{tr}(B)=1+5=6,tr(B)=1+5=6, det⁡(B)=1⋅5−3⋅1=2.\det(B)=1\cdot 5-3\cdot 1=2.det(B)=1⋅5−3⋅1=2.

Therefore the characteristic polynomial is λ2−6λ+2.\lambda^2-6\lambda+2.λ2−6λ+2.

By Cayley-Hamilton, B2−6B+2I=O.B^2-6B+2I=O.B2−6B+2I=O.


  1. Convert this into an equation for AAA

Since B=A2B=A^2B=A2, substitute into the above: (A2)2−6A2+2I=O,(A^2)^2-6A^2+2I=O,(A2)2−6A2+2I=O, that is, A4−6A2+2I=O.A^4-6A^2+2I=O.A4−6A2+2I=O.

Because CCC is similar to AAA, CCC satisfies the same polynomial: C4−6C2+2I=O.C^4-6C^2+2I=O.C4−6C2+2I=O.

Compare with C4+αC2+βI=O.C^4+\alpha C^2+\beta I=O.C4+αC2+βI=O. So, α=−6,β=2.\alpha=-6, \qquad \beta=2.α=−6,β=2.


  1. Compute 2β−α2\beta-\alpha2β−α

2β−α=2(2)−(−6)=4+6=10.2\beta-\alpha=2(2)-(-6)=4+6=10.2β−α=2(2)−(−6)=4+6=10.


  1. Check options

The correct option is: 10\boxed{10}10​ which is Option B.


  1. Comparison with stored answer

Stored correct answer: B

Our derived answer is also B. So they agree.

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