Question Number 83213 by ~blr237~ last updated on 28/Feb/20
$${Prove}\:\:{that}\:\:\forall\:\theta\in\mathbb{R}\:\:\:\underset{{n}=\mathrm{1}} {\overset{\infty} {\sum}}\mathrm{3}^{{n}−\mathrm{1}} {sin}^{\mathrm{3}} \left(\frac{\theta}{\mathrm{3}^{{n}} }\right)=\:\frac{\theta−{sin}\theta}{\mathrm{4}}\: \\ $$