Let A 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 $\mathrm{A}\left[\begin{array}{l}1 \\ 1 \\ 1\end{array}\right]=\left[\begin{array}{c}1 \\ 4 \\ -5\end{array}\right], \mathrm{A}\left[\begin{array}{l}1 \\ 2 \\ 1\end{array}\right]=\left[\begin{array}{c}0 \\ 4 \\ -8\end{array}\right]$, and $\operatorname{det}(\operatorname{adj}(2(\mathrm{~A}+\mathrm{I})))=2^\alpha 3^\beta 5^\gamma, \alpha, \beta, \gamma \in N$, then $\alpha^2+\beta^2+\gamma^2$ is
Solution
<p>$$\begin{aligned}
& X^T A X=0 \\
& (x y z)\left(\begin{array}{lll}
a_1 & a_2 & a_3 \\
b_1 & b_2 & b_3 \\
c_1 & c_2 & c_3
\end{array}\right)\left(\begin{array}{l}
x \\
y \\
z
\end{array}\right)=0 \\
& (x y z)\left(\begin{array}{l}
a_1 x+a_2 y+a_3 z \\
b_1 x+b_2 y+b_3 z \\
c_1 x+c_2 y+c_3 z
\end{array}\right)=0
\end{aligned}$$</p>
<p>$$\begin{aligned}
& x\left(a_1 x+a_2 y+a_3 z\right)+y\left(b_1 x+b_2 y+b_3 z\right) \\
& +z\left(c_1 x+c_2 y+c_3 z\right)=0 \\
& a_1=0, b_2=0 c_3=0 \\
& a_2+b_1=0, a_3+c_1=0, b_3=c_2=0
\end{aligned}$$</p>
<p>$$\begin{aligned}
& A=\text { skew symm matrix } \\
& A=\left(\begin{array}{ccc}
0 & x & y \\
-x & 0 & z \\
-y & -z & 0
\end{array}\right) ; \quad A=\left(\begin{array}{l}
1 \\
1 \\
1
\end{array}\right)=\left(\begin{array}{l}
1 \\
4 \\
-5
\end{array}\right) \\
& \Rightarrow A=\left(\begin{array}{ccc}
0 & x & y \\
-x & 0 & z \\
-y & -z & 0
\end{array}\right)\left(\begin{array}{l}
1 \\
1 \\
1
\end{array}\right)=\left(\begin{array}{l}
1 \\
4 \\
-5
\end{array}\right)
\end{aligned}$$</p>
<p>$$\begin{aligned}
& x+y=1 \\
& -x+z=4 \\
& y+z=5 \\
& \left(\begin{array}{ccc}
0 & x & y \\
-x & 0 & z \\
-y & -z & 0
\end{array}\right)\left(\begin{array}{l}
1 \\
2 \\
1
\end{array}\right)=\left(\begin{array}{l}
1 \\
4 \\
-8
\end{array}\right)
\end{aligned}$$</p>
<p>$$\begin{array}{ll}
2 x+y=0 & x=-1 \\
-x+z=4 & y=2 \\
-y-2 z=-8 & z=3
\end{array}$$</p>
<p>$$\begin{aligned}
& A=\left(\begin{array}{ccc}
0 & -1 & 2 \\
1 & 0 & 3 \\
-2 & -3 & 0
\end{array}\right) \\
& 2(\mathrm{~A}+\mathrm{I})=\left(\begin{array}{ccc}
2 & -2 & 4 \\
2 & 2 & 6 \\
-2 & -6 & 2
\end{array}\right) \\
& 2(\mathrm{~A}+\mathrm{I})=120 \Rightarrow \operatorname{det}|\operatorname{adi}(2(\mathrm{~A}+\mathrm{I}))| \\
& =120^2=2^6 \cdot 3^2 \cdot 5^2 \\
& \alpha=6, \beta=2, \gamma=2
\end{aligned}$$</p>
About this question
Subject: Mathematics · Chapter: Matrices and Determinants · Topic: Types of Matrices and Operations
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