The following functions are solutions to a real constant coefficient homogeneous scalar ordinary differential equation. (i) Determine the least possible order of the differential equation, and (ii) write down an appropriate differential equation.
(a) $e^{-x} \sin 3 x$, (b) $x \sin x$,
(c) $1+x e^{-x} \cos 2 x$,
(d) $\sin x+\cos 2 x$,
(e) $\sin x+x^2 \cos x$.