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Question

Let A be a $3 \times 3$ matrix such that
$A^T \begin{bmatrix} 1 \\ 0 \\ 1 \end{bmatrix} = \begin{bmatrix} 5 \\ 2 \\ 2 \end{bmatrix}$, $A^T \begin{bmatrix} 0 \\ 0 \\ 1 \end{bmatrix} = \begin{bmatrix} 3 \\ 1 \\ 1 \end{bmatrix}$, $A \begin{bmatrix} 1 \\ 0 \\ 1 \end{bmatrix} = \begin{bmatrix} 3 \\ 4 \\ 4 \end{bmatrix}$ and $A \begin{bmatrix} 0 \\ 0 \\ 1 \end{bmatrix} = \begin{bmatrix} 1 \\ 3 \\ 1 \end{bmatrix}$.
If $\text{det}(A) = 1$, then $\text{det}(\text{adj}(A^2 + A))$ is equal to:

The correct answer is
64

The problem asks for the determinant of the adjugate of the matrix $A^2 + A$, given certain properties of matrix $A$ and its determinant.

Matrix A Determination

First, let's determine the matrix $A$. Let $A = \begin{bmatrix} a_{11} & a_{12} & a_{13} \\ a_{21} & a_{22} & a_{23} \\ a_{31} & a_{32} & a_{33} \end{bmatrix}$.

  • From $A \begin{bmatrix} 0 \\ 0 \\ 1 \end{bmatrix} = \begin{bmatrix} 1 \\ 3 \\ 1 \end{bmatrix}$, the third column of $A$ is $\begin{bmatrix} 1 \\ 3 \\ 1 \end{bmatrix}$. So, $a_{13}=1, a_{23}=3, a_{33}=1$.
  • From $A^T \begin{bmatrix} 0 \\ 0 \\ 1 \end{bmatrix} = \begin{bmatrix} 3 \\ 1 \\ 1 \end{bmatrix}$, the third column of $A^T$ is $\begin{bmatrix} 3 \\ 1 \\ 1 \end{bmatrix}$. This implies the third row of $A$ is $\begin{bmatrix} 3 & 1 & 1 \end{bmatrix}$. So, $a_{31}=3, a_{32}=1, a_{33}=1$. (This is consistent with the previous point for $a_{33}$).
  • The matrix A now looks like: $A = \begin{bmatrix} a_{11} & a_{12} & 1 \\ a_{21} & a_{22} & 3 \\ 3 & 1 & 1 \end{bmatrix}$.
  • Using $A \begin{bmatrix} 1 \\ 0 \\ 1 \end{bmatrix} = \begin{bmatrix} 3 \\ 4 \\ 4 \end{bmatrix}$: $\begin{bmatrix} a_{11} & a_{12} & 1 \\ a_{21} & a_{22} & 3 \\ 3 & 1 & 1 \end{bmatrix} \begin{bmatrix} 1 \\ 0 \\ 1 \end{bmatrix} = \begin{bmatrix} a_{11}+1 \\ a_{21}+3 \\ 3+1 \end{bmatrix} = \begin{bmatrix} 3 \\ 4 \\ 4 \end{bmatrix}$. This gives $a_{11}+1=3 \implies a_{11}=2$ and $a_{21}+3=4 \implies a_{21}=1$.
  • Using $A^T \begin{bmatrix} 1 \\ 0 \\ 1 \end{bmatrix} = \begin{bmatrix} 5 \\ 2 \\ 2 \end{bmatrix}$: $A^T = \begin{bmatrix} 2 & 1 & 3 \\ a_{12} & a_{22} & 1 \\ 1 & 3 & 1 \end{bmatrix}$. $\begin{bmatrix} 2 & 1 & 3 \\ a_{12} & a_{22} & 1 \\ 1 & 3 & 1 \end{bmatrix} \begin{bmatrix} 1 \\ 0 \\ 1 \end{bmatrix} = \begin{bmatrix} 2+3 \\ a_{12}+1 \\ 1+1 \end{bmatrix} = \begin{bmatrix} 5 \\ 2 \\ 2 \end{bmatrix}$. This gives $a_{12}+1=2 \implies a_{12}=1$.
  • The matrix A is $A = \begin{bmatrix} 2 & 1 & 1 \\ 1 & a_{22} & 3 \\ 3 & 1 & 1 \end{bmatrix}$.
  • We are given $\text{det}(A) = 1$. Calculate the determinant of A in terms of $a_{22}$: $\text{det}(A) = 2(a_{22} \times 1 - 3 \times 1) - 1(1 \times 1 - 3 \times 3) + 1(1 \times 1 - a_{22} \times 3)$ $\text{det}(A) = 2(a_{22}-3) - 1(1-9) + 1(1-3a_{22})$ $\text{det}(A) = 2a_{22}-6 + 8 + 1-3a_{22} = 3 - a_{22}$.
  • Setting $\text{det}(A)=1$: $1 = 3 - a_{22} \implies a_{22} = 2$.
  • So, $A = \begin{bmatrix} 2 & 1 & 1 \\ 1 & 2 & 3 \\ 3 & 1 & 1 \end{bmatrix}$.

Calculate $A^2 + A$

First, calculate $A^2$:

$A^2 = \begin{bmatrix} 2 & 1 & 1 \\ 1 & 2 & 3 \\ 3 & 1 & 1 \end{bmatrix} \begin{bmatrix} 2 & 1 & 1 \\ 1 & 2 & 3 \\ 3 & 1 & 1 \end{bmatrix} = \begin{bmatrix} 4+1+3 & 2+2+1 & 2+3+1 \\ 2+2+9 & 1+4+3 & 1+6+3 \\ 6+1+3 & 3+2+1 & 3+3+1 \end{bmatrix} = \begin{bmatrix} 8 & 5 & 6 \\ 13 & 8 & 10 \\ 10 & 6 & 7 \end{bmatrix}$

Now, calculate $A^2 + A$:

$A^2 + A = \begin{bmatrix} 8 & 5 & 6 \\ 13 & 8 & 10 \\ 10 & 6 & 7 \end{bmatrix} + \begin{bmatrix} 2 & 1 & 1 \\ 1 & 2 & 3 \\ 3 & 1 & 1 \end{bmatrix} = \begin{bmatrix} 10 & 6 & 7 \\ 14 & 10 & 13 \\ 13 & 7 & 8 \end{bmatrix}$

Calculate Determinant of $A^2 + A$

Let $M = A^2 + A = \begin{bmatrix} 10 & 6 & 7 \\ 14 & 10 & 13 \\ 13 & 7 & 8 \end{bmatrix}$.

Calculate the determinant of $M$:

$\text{det}(M) = 10 \begin{vmatrix} 10 & 13 \\ 7 & 8 \end{vmatrix} - 6 \begin{vmatrix} 14 & 13 \\ 13 & 8 \end{vmatrix} + 7 \begin{vmatrix} 14 & 10 \\ 13 & 7 \end{vmatrix}$

$\text{det}(M) = 10(10 \times 8 - 13 \times 7) - 6(14 \times 8 - 13 \times 13) + 7(14 \times 7 - 10 \times 13)$

$\text{det}(M) = 10(80 - 91) - 6(112 - 169) + 7(98 - 130)$

$\text{det}(M) = 10(-11) - 6(-57) + 7(-32)$

$\text{det}(M) = -110 + 342 - 224 = 342 - 334 = 8$.

Calculate Determinant of Adjugate Matrix

We need to find $\text{det}(\text{adj}(A^2 + A)) = \text{det}(\text{adj}(M))$.

For an $n \times n$ matrix $M$, the determinant of its adjugate is given by the formula $\text{det}(\text{adj}(M)) = (\text{det}(M))^{n-1}$.

Here, $M$ is a $3 \times 3$ matrix, so $n=3$.

$\text{det}(\text{adj}(M)) = (\text{det}(M))^{3-1} = (\text{det}(M))^2$.

Substituting $\text{det}(M) = 8$:

$\text{det}(\text{adj}(A^2 + A)) = (8)^2 = 64$.

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Similar Questions

  1. Let A be a $3 \times 3$ matrix such that $A + A^T = O$. If $A\begin{bmatrix} 1 \\ -1 \\ 0 \end{bmatrix} = \begin{bmatrix} 3 \\ 3 \\ 2 \end{bmatrix}$, $A^2\begin{bmatrix} 1 \\ -1 \\ 0 \end{bmatrix} = \begin{bmatrix} -3 \\ 19 \\ -24 \end{bmatrix}$ and $\det(adj(2 \ adj(A + I))) = (2)^\alpha \cdot (3)^\beta \cdot (11)^\gamma$, $\alpha, \beta, \gamma$ are non-negative integers, then $\alpha + \beta + \gamma$ is equal to _________
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Important Questions from Algebra

  1. Let A be a $3 \times 3$ matrix such that $A + A^T = O$. If $A\begin{bmatrix} 1 \\ -1 \\ 0 \end{bmatrix} = \begin{bmatrix} 3 \\ 3 \\ 2 \end{bmatrix}$, $A^2\begin{bmatrix} 1 \\ -1 \\ 0 \end{bmatrix} = \begin{bmatrix} -3 \\ 19 \\ -24 \end{bmatrix}$ and $\det(adj(2 \ adj(A + I))) = (2)^\alpha \cdot (3)^\beta \cdot (11)^\gamma$, $\alpha, \beta, \gamma$ are non-negative integers, then $\alpha + \beta + \gamma$ is equal to _________
  2. Let $\alpha = \frac{-1 + i\sqrt{3}}{2}$ and $\beta = \frac{-1 - i\sqrt{3}}{2}$, $i = \sqrt{-1}$. If $(7 - 7\alpha + 9\beta)^{20} + (9 + 7\alpha - 7\beta)^{20} + (-7 + 9\alpha + 7\beta)^{20} + (14 + 7\alpha + 7\beta)^{20} = m^{10}$, then $m$ is _________
  3. Let ABC be a triangle. Consider four points $p_1, p_2, p_3, p_4$ on the side AB, five points $p_5, p_6, p_7, p_8, p_9$ on the side BC, and four points $p_{10}, p_{11}, p_{12}, p_{13}$ on the side AC. None of these points is a vertex of the triangle ABC. Then the total number of pentagons, that can be formed by taking all the vertices from the points $p_1, p_2, ..., p_{13}$, is _________
  4. If $X = \begin{bmatrix} x \\ y \\ z \end{bmatrix}$ is a solution of the system of equations $AX = B$, where $\text{adj } A = \begin{bmatrix} 4 & 2 & 2 \\ -5 & 0 & 5 \\ 1 & -2 & 3 \end{bmatrix}$ and $B = \begin{bmatrix} 4 \\ 0 \\ 2 \end{bmatrix}$, then $|x + y + z|$ is equal to :
  5. Let $C_r$ denote the coefficient of $x^r$ in the binomial expansion of $(1 + x)^n$, $n \in \mathbb{N}, 0 \leq r \leq n$. If $P_n = C_0 - C_1 + \frac{2^2}{3} C_2 - \frac{2^3}{4} C_3 + \dots + \frac{(-2)^n}{n+1} C_n$, then the value of $\sum_{n=1}^{25} \frac{1}{P_{2n}}$ equals.
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