(b) Here $P_{1} = i^{2} \times \frac{r}{3},\ P_{2} = i^{2} \times 3r$,
$P_{3} = i^{2}\left( \frac{r}{2} + r \right) = \frac{3}{2}i^{2}r$
and $P_{4} = i^{2}\left\lbrack \frac{2r \times 2r}{2r + 2r} \right\rbrack = i^{2}r$
So it is obvious that $P_{2} > P_{3} > P_{4} > P_{1}$