Solution
= Solution
Let $r_0=(3Nk_BT/(4\pi P_0))^{1/3}$ be the zero-tension radius. From
$P_0r^3+2tr^2=P_0r_0^3$, put $r=r_0+\delta$ and retain first order:
$$
3P_0r_0^2\delta+2tr_0^2=0,\qquad
r=r_0-\frac{2t}{3P_0}+O(t^2).
$$
= Solution
Let $r_0=(3Nk_BT/(4\pi P_0))^{1/3}$ be the zero-tension radius. From
$P_0r^3+2tr^2=P_0r_0^3$, put $r=r_0+\delta$ and retain first order:
$$
3P_0r_0^2\delta+2tr_0^2=0,\qquad
r=r_0-\frac{2t}{3P_0}+O(t^2).
$$