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(i)
$V = PQ \times QR \times \mathrm{height}$

$= (2n-2x)(n-2x)x$

$= 2n^2x-6nx^2+4x^3$

(ii)
$\frac{dV}{dx}=2n^2 - 12nx +12x^2$

Let $\frac{dV}{dx}=0$

$\Rightarrow 6x^2 -6nx + n^2 = 0$

$x = (\frac{3 \pm \sqrt{3}}{6})n$

Since $2x \textless n, ~ x \textless \frac{n}{2} \Rightarrow x = (\frac{3- \sqrt{3}}{6})n$

We reject $x=\frac{3+\sqrt{3}}{6}n$ since $\frac{3+\sqrt{3}}{6}n > \frac{n}{2}$

$\therefore$ Required stationary value of $x = \frac{3-\sqrt{3}}{6}n$