Question Number 32935 by abdo imad last updated on 06/Apr/18
$${let}\:{give}\:\mid{x}\mid<\mathrm{1}\:\:{prove}\:{that} \\ $$$$\sum_{{n}=\mathrm{1}} ^{\infty} \:{x}^{{n}} {sin}\left({n}\theta\right)\:=\:\:\frac{{x}\:{sin}\theta}{\mathrm{1}−\mathrm{2}{x}\:{cos}\theta\:+{x}^{\mathrm{2}} }\:. \\ $$