Question
Let be a $3 \times 3$ matrix such that $\mathrm{X}^{\mathrm{T}} \mathrm{AX}=\mathrm{O}$ for all nonzero $3 \times 1$ matrices $X=\left[\begin{array}{l}x \\ y \\ z\end{array}\right]$. If $\mathrm{A}\left[\begin{array}{l}1 \\ 1 \\ 1\end{array}\right]=\left[\begin{array}{l}1 \\ 4 \\ -5\end{array}\right], \mathrm{A}\left[\begin{array}{l}1 \\ 2 \\ 1\end{array}\right]=\left[\begin{array}{l}0 \\ 4 \\ -8\end{array}\right]$, and
$\operatorname{det}(\operatorname{adj}(2(\mathrm{~A}+\mathrm{I})))=2^{\alpha} 3^{\beta} 5^{\gamma}, \alpha, \beta, \gamma, \in \mathrm{N}$, then $\alpha^{2}+\beta^{2}+\gamma^{2}$ is

Answer

(44)
Sol. $X^{T} A X=0$
(xyz) $\left(\begin{array}{lll}a_{1} & a_{2} & a_{3} \\ b_{1} & b_{2} & b_{3} \\ c_{1} & c_{2} & c_{3}\end{array}\right)\left(\begin{array}{l}x \\ y \\ z\end{array}\right)=0$
$(x y z)\left(\begin{array}{l}a_{1} x+a_{2} y+a_{3} z \\ b_{1} x+b_{2} y+b_{3} z \\ c_{1} x+c_{2} y+c_{3} z\end{array}\right)=0$
$x\left(a_{1} x+a_{2} y+a_{3} z\right)+y\left(b_{1} x+b_{2} y+b_{3} z\right)$
$+z\left(c_{1} x+c_{2} y+c_{3} z\right)=0$
$a_{1}=0, b_{2}=0 c_{3}=0$
$\mathrm{a}_{2}+\mathrm{b}_{1}=0, \mathrm{a}_{3}+\mathrm{c}_{1}=0, \mathrm{~b}_{3}=\mathrm{c}_{2}=0$
$\mathrm{A}=$ skew symm matrix
$\mathrm{A}=\left(\begin{array}{ccc}0 & \mathrm{x} & \mathrm{y} \\ -\mathrm{x} & 0 & \mathrm{z} \\ -\mathrm{y} & -\mathrm{z} & 0\end{array}\right) ; \quad \mathrm{A}=\left(\begin{array}{l}1 \\ 1 \\ 1\end{array}\right)=\left(\begin{array}{l}1 \\ 4 \\ -5\end{array}\right)$
$\Rightarrow A=\left(\begin{array}{ccc}0 & x & y \\ -x & 0 & z \\ -y & -z & 0\end{array}\right)\left(\begin{array}{l}1 \\ 1 \\ 1\end{array}\right)=\left(\begin{array}{l}1 \\ 4 \\ -5\end{array}\right)$
$x+y=1$
$-x+z=4$
$y+z=5$
$\left(\begin{array}{ccc}0 & x & y \\ -x & 0 & z \\ -y & -z & 0\end{array}\right)\left(\begin{array}{l}1 \\ 2 \\ 1\end{array}\right)=\left(\begin{array}{l}1 \\ 4 \\ -8\end{array}\right)$
$2 x+y=0 \quad x=-1$
$-x+z=4 \quad y=2$
$-\mathrm{y}-2 \mathrm{z}=-8 \quad \mathrm{z}=3$
$\mathrm{A}=\left(\begin{array}{ccc}0 & -1 & 2 \\ 1 & 0 & 3 \\ -2 & -3 & 0\end{array}\right)$
$2(\mathrm{~A}+\mathrm{I})=\left(\begin{array}{ccc}2 & -2 & 4 \\ 2 & 2 & 6 \\ -2 & -6 & 2\end{array}\right)$
$2(\mathrm{~A}+\mathrm{I})=120 \Rightarrow \operatorname{det}|\operatorname{adi}(2(\mathrm{~A}+\mathrm{I}))|$
$=120^{2}=2^{6} \cdot 3^{2} \cdot 5^{2}$
$\alpha=6, \beta=2, \gamma=2$

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