- Home
- Standard 12
- Mathematics
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