3 and 4 .Determinants and Matrices
hard

Let $S$ be the set containing all $3 \times 3$ matrices with entries from $\{-1,0,1\}$. The total number of matrices $A \in S$ such that the sum of all the diagonal elements of $A ^{ T } A$ is $6$ is.

A

$5376$

B

$5377$

C

$5369$

D

$5362$

(JEE MAIN-2022)

Solution

$\operatorname{Tr}\left(A A^{T}\right)=6$

$AA ^{ T }=\left[\begin{array}{lll} a & d & g \\ b & e & h \\ c & f & i \end{array}\right]\left[\begin{array}{lll} a & b & c \\d & e & f \\ g & h & i \end{array}\right]$

Now given $a^{2}+d^{2}+g^{2}+b^{2}+e^{2}+h^{2}+c^{2}+f^{2}+i^{2}=6$

$={ }^{9} C_{3} \times 2^{6}$

$=5376$

Standard 12
Mathematics

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