d
(d)\(F = \frac{{ - dU}}{{dx}} \Rightarrow dU = - F\,\,dx\)
\( \Rightarrow U = - \int_0^x {( - Kx\, + \,a{x^3})dx} \)\( = \frac{{k{x^2}}}{2} - \frac{{a{x^4}}}{4}\)
\(\therefore \) We get \(U = 0\) at \(x = 0 \) and \(x =\) \(\sqrt {2k/a} \)
and also \(U =\) negative for \(x > \sqrt {2k/a} \).
So \(F = 0\) at \(x = 0\)
i.e. slope of \(U -x\) graph is zero at \(x = 0.\)