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3 and 4 .Determinants and Matrices
normal
If $P$ is a $3 \times 3$ matrix such that $P^{\top}=2 P+I$, where $P^{\top}$ is the transpose of $P$ and $I$ is the $3 \times 3$ identity matrix, then there exists a column matrix $X=\left[\begin{array}{l}x \\ y \\ z\end{array}\right] \neq\left[\begin{array}{l}0 \\ 0 \\ 0\end{array}\right]$ such that
A
$PX =\left[\begin{array}{l}0 \\ 0 \\ 0\end{array}\right]$
B
$P X=X$
C
$P X=2 X$
D
$P X=-X$
(IIT-2012)
Solution
$P ^{\top}=2 P + I $
$\Rightarrow \quad\left(P^{\top}\right)^{\top}=(2 P+I)^{\top} $
$\Rightarrow \quad P=2 P ^{\top}+ I $
$\Rightarrow \quad P=2(2 P + I )+ I $
$\Rightarrow \quad 3 P =-3 I \quad \Rightarrow \quad P =- I $
$\Rightarrow \quad P X=-I X=-X $
Standard 12
Mathematics