Question Number 11403 by JAZAR last updated on 24/Mar/17
$${how}\:{can}\:{demonstred}\:{that}\: \\ $$$${cos}\left(\mathrm{2}{x}\right)−{sin}\left(\mathrm{2}{x}\right)−\mathrm{1}=−{cos}^{\mathrm{2}} {x} \\ $$
Commented by sm3l2996 last updated on 24/Mar/17
$$\mathrm{I}\:\mathrm{think}\:\mathrm{that}\:\mathrm{equation}\:\mathrm{is}\:\mathrm{not}\:\mathrm{correct} \\ $$$$\mathrm{because}\:\mathrm{cos}\left(\mathrm{2}\pi\right)−\mathrm{sin}\left(\mathrm{2}\pi\right)−\mathrm{1}=\mathrm{0}\neq−\mathrm{cos}^{\mathrm{2}} \left(\pi\right) \\ $$