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Matrices and Determinants question

2023 · 8 Apr · Shift 1 · Q24
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  5. /2023 · 8 Apr · Shift 1 · Q24

Matrices and Determinants question

2023 · 8 Apr · Shift 1 · Q24

JEE MainMathematicsMatrices and DeterminantsMCQ+4 / −1
Let A=[21012−10−12]A=\left[\begin{array}{ccc}2 & 1 & 0 \\ 1 & 2 & -1 \\ 0 & -1 & 2\end{array}\right]A=​210​12−1​0−12​​. If ∣adj⁡(adj⁡(adj⁡2A))∣=(16)n|\operatorname{adj}(\operatorname{adj}(\operatorname{adj} 2 A))|=(16)^{n}∣adj(adj(adj2A))∣=(16)n, then nnn is equal to :
  1. A
    9
  2. B
    8
  3. C
    10
  4. D
    12
View written solutionFree

Correct answer: C

  1. Use the determinant formula for adjugate

For an m×mm \times mm×m matrix MMM,

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

Here, the matrix is 3×33\times 33×3, so

det⁡(adj⁡M)=(det⁡M)2.\det(\operatorname{adj} M)=\big(\det M\big)^2.det(adjM)=(detM)2.

Also, for a scalar multiple:

det⁡(kA)=k3det⁡(A)for a 3×3 matrix.\det(kA)=k^3\det(A) \quad \text{for a }3\times 3\text{ matrix}.det(kA)=k3det(A)for a 3×3 matrix.
  1. First compute det⁡(A)\det(A)det(A)

Given

A=[21012−10−12].A=\begin{bmatrix}2&1&0\\1&2&-1\\0&-1&2\end{bmatrix}.A=​210​12−1​0−12​​.

Expand along the first row:

det⁡(A)=2∣2−1−12∣−1∣1−102∣+0.\det(A)=2\begin{vmatrix}2&-1\\-1&2\end{vmatrix}-1\begin{vmatrix}1&-1\\0&2\end{vmatrix}+0.det(A)=2​2−1​−12​​−1​10​−12​​+0.

Now,

∣2−1−12∣=4−1=3,\begin{vmatrix}2&-1\\-1&2\end{vmatrix}=4-1=3,​2−1​−12​​=4−1=3,

and

∣1−102∣=2.\begin{vmatrix}1&-1\\0&2\end{vmatrix}=2.​10​−12​​=2.

So,

det⁡(A)=2(3)−1(2)=6−2=4.\det(A)=2(3)-1(2)=6-2=4.det(A)=2(3)−1(2)=6−2=4.

Hence,

det⁡(2A)=23det⁡(A)=8⋅4=32.\det(2A)=2^3\det(A)=8\cdot 4=32.det(2A)=23det(A)=8⋅4=32.
  1. Apply adjugate repeatedly

Let

M0=2A.M_0=2A.M0​=2A.

Then

det⁡(M0)=32.\det(M_0)=32.det(M0​)=32.

Now define

M1=adj⁡(M0),M2=adj⁡(M1),M3=adj⁡(M2).M_1=\operatorname{adj}(M_0),\quad M_2=\operatorname{adj}(M_1),\quad M_3=\operatorname{adj}(M_2).M1​=adj(M0​),M2​=adj(M1​),M3​=adj(M2​).

We need

det⁡(M3)=∣adj⁡(adj⁡(adj⁡(2A)))∣.\det(M_3)=\left|\operatorname{adj}(\operatorname{adj}(\operatorname{adj}(2A)))\right|.det(M3​)=∣adj(adj(adj(2A)))∣.

Since each time for a 3×33\times 33×3 matrix,

det⁡(adj⁡M)=(det⁡M)2,\det(\operatorname{adj} M)=(\det M)^2,det(adjM)=(detM)2,

we get:

  • First adjugate:
det⁡(M1)=322.\det(M_1)=32^2.det(M1​)=322.
  • Second adjugate:
det⁡(M2)=(322)2=324.\det(M_2)=(32^2)^2=32^4.det(M2​)=(322)2=324.
  • Third adjugate:
det⁡(M3)=(324)2=328.\det(M_3)=(32^4)^2=32^8.det(M3​)=(324)2=328.

Thus,

∣adj⁡(adj⁡(adj⁡(2A)))∣=328.\left|\operatorname{adj}(\operatorname{adj}(\operatorname{adj}(2A)))\right|=32^8.∣adj(adj(adj(2A)))∣=328.
  1. Express in the form (16)n(16)^n(16)n

Since

32=25,16=24,32=2^5, \qquad 16=2^4,32=25,16=24,

we have

328=(25)8=240=(24)10=1610.32^8=(2^5)^8=2^{40}=(2^4)^{10}=16^{10}.328=(25)8=240=(24)10=1610.

Therefore,

n=10.n=10.n=10.
  1. Check with options

Option C\boxed{\text{C}}C​ is correct.

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