Show that the solution of of the initial value problem \(u(0)=1\) of the differential equation
\[
\left(\frac{d u}{d x}\right)^{2}=\frac{2}{9} \frac{x^{2}}{u^{3}}
\]
is given by
\[
u(x)=\left(\frac{5}{6 \sqrt{2}}\left(x^{2}-1\right)+1\right)^{2 / 5}
\]