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JEE Advance 2012
paper 2
Question
If P is a $3 \times 3$ matrix such that $\mathrm{P}^{\top}=2 \mathrm{P}+\mathrm{I}$, where $\mathrm{P}^{\top}$ is the transpose of P and I is the $3 \times 3$ identity matrix then there exists a column matrix $X=\left[\begin{array}{c}x \\ y \\ z\end{array}\right] \neq\left[\begin{array}{l}0 \\ 0 \\ 0\end{array}\right]$ such that
Select the correct option:
A
$\mathrm{PX}=\left[\begin{array}{l}0 \\ 0 \\ 0\end{array}\right]$
B
$P X=X$
C
$P X=2 X$
D
$P X=-X$
✓ Correct! Well done.
✗ Incorrect. Try again or view the solution.
Solution
$\begin{array}{llll} & P^{\top}=2 P+I & & \\ \Rightarrow & \left(P^{\top}\right)^{\top}=(2 P+I)^{\top} & & \\ \Rightarrow & P=2 P^{\top}+I & & \\ \Rightarrow & P=2(2 P+I)+I & & \\ \Rightarrow & 3 P=-3 I & & P=-I \\ \Rightarrow & P X=-I X=-X & & \end{array}$
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