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JEE Main 20265 AprilMorning Shift
Exam
JEE Main
Year
2026
Date
5 April
Shift
Morning Shift
Subject
Mathematics
Chapter
Matrices

JEE Main 2026 · 5 April · Morning Shift · Mathematics · Matrices

Let A be a 3 × 3 matrix such that A^T bmatrix 1 0 1 bmatrix =...

Let AA be a 3×33 \times 3 matrix such that AT[101]=[522]A^T \begin{bmatrix}1\\0\\1\end{bmatrix} = \begin{bmatrix}5\\2\\2\end{bmatrix}, AT[001]=[311]A^T \begin{bmatrix}0\\0\\1\end{bmatrix} = \begin{bmatrix}3\\1\\1\end{bmatrix}, A[101]=[344]A \begin{bmatrix}1\\0\\1\end{bmatrix} = \begin{bmatrix}3\\4\\4\end{bmatrix} and A[001]=[131]A \begin{bmatrix}0\\0\\1\end{bmatrix} = \begin{bmatrix}1\\3\\1\end{bmatrix}. If det⁡(A)=1\det(A) = 1, then det⁡(adj⁡(A2+A))\det(\operatorname{adj}(A^2 + A)) is equal to:

Options

  1. A1616
  2. B2525
  3. C4949
  4. D6464

Correct answer

D. 6464

Step-by-step solution

Let C1,C2,C3C_1, C_2, C_3 be the columns of AA and R1,R2,R3R_1, R_2, R_3 be the rows of AA.

From the given equations, we have:
A[001]=C3=[131]A \begin{bmatrix}0\\0\\1\end{bmatrix} = C_3 = \begin{bmatrix}1\\3\\1\end{bmatrix}

A[101]=C1+C3=[344]⇒C1=[213]A \begin{bmatrix}1\\0\\1\end{bmatrix} = C_1 + C_3 = \begin{bmatrix}3\\4\\4\end{bmatrix} \Rightarrow C_1 = \begin{bmatrix}2\\1\\3\end{bmatrix}

Similarly, for the transpose ATA^T, the columns correspond to the rows of AA:
AT[001]=R3T=[311]⇒R3=[311]A^T \begin{bmatrix}0\\0\\1\end{bmatrix} = R_3^T = \begin{bmatrix}3\\1\\1\end{bmatrix} \Rightarrow R_3 = \begin{bmatrix}3 & 1 & 1\end{bmatrix}

AT[101]=R1T+R3T=[522]⇒R1T=[211]⇒R1=[211]A^T \begin{bmatrix}1\\0\\1\end{bmatrix} = R_1^T + R_3^T = \begin{bmatrix}5\\2\\2\end{bmatrix} \Rightarrow R_1^T = \begin{bmatrix}2\\1\\1\end{bmatrix} \Rightarrow R_1 = \begin{bmatrix}2 & 1 & 1\end{bmatrix}

Using the elements of C1,C3,R1,C_1, C_3, R_1, and R3R_3, we can construct the matrix AA with an unknown central element bb:
A=[2111b3311]A = \begin{bmatrix} 2 & 1 & 1 \\ 1 & b & 3 \\ 3 & 1 & 1 \end{bmatrix}

We are given that det⁡(A)=1\det(A) = 1. Expanding along the first row:
det⁡(A)=2(b−3)−1(1−9)+1(1−3b)=1\det(A) = 2(b - 3) - 1(1 - 9) + 1(1 - 3b) = 1

2b−6+8+1−3b=12b - 6 + 8 + 1 - 3b = 1

3−b=1⇒b=23 - b = 1 \Rightarrow b = 2

Thus, the matrix AA is:
A=[211123311]A = \begin{bmatrix} 2 & 1 & 1 \\ 1 & 2 & 3 \\ 3 & 1 & 1 \end{bmatrix}

We need to find det⁡(adj⁡(A2+A))\det(\operatorname{adj}(A^2 + A)). Using the properties of determinants and adjoints:
det⁡(adj⁡(A2+A))=(det⁡(A2+A))2=(det⁡(A)⋅det⁡(A+I))2\det(\operatorname{adj}(A^2 + A)) = (\det(A^2 + A))^2 = (\det(A) \cdot \det(A + I))^2

First, find A+IA + I:
A+I=[311133312]A + I = \begin{bmatrix} 3 & 1 & 1 \\ 1 & 3 & 3 \\ 3 & 1 & 2 \end{bmatrix}

Now, calculate det⁡(A+I)\det(A + I):
det⁡(A+I)=3(6−3)−1(2−9)+1(1−9)\det(A + I) = 3(6 - 3) - 1(2 - 9) + 1(1 - 9)

det⁡(A+I)=3(3)−1(−7)+1(−8)=9+7−8=8\det(A + I) = 3(3) - 1(-7) + 1(-8) = 9 + 7 - 8 = 8

Since det⁡(A)=1\det(A) = 1, we have:
det⁡(A2+A)=1×8=8\det(A^2 + A) = 1 \times 8 = 8

Finally, for a 3×33 \times 3 matrix:
det⁡(adj⁡(A2+A))=83−1=82=64\det(\operatorname{adj}(A^2 + A)) = 8^{3-1} = 8^2 = 64

Answer: 6464
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