It is given that $f(x)=\ln (1+\sin 3 x)$.

(i) Show that $\mathrm{f}^{\prime \prime}(x)=\frac{k}{1+\sin 3 x}$, where $k$ is a constant to be found.

[3]

(ii) Hence find the first three non-zero terms of the Maclaurin expansion of $f(x)$.

[4]