Question Number 84202 by 1406 last updated on 10/Mar/20
$${prove}\: \\ $$$${cos}^{\mathrm{4}} {x}−{sin}^{\mathrm{4}} {x}={cos}\mathrm{22}{x} \\ $$
Answered by john santu last updated on 10/Mar/20
$$\left(\mathrm{cos}\:^{\mathrm{2}} \mathrm{x}−\mathrm{sin}\:^{\mathrm{2}} \mathrm{x}\right)\left(\mathrm{cos}\:^{\mathrm{2}} \mathrm{x}+\mathrm{sin}\:^{\mathrm{2}} \mathrm{x}\right) \\ $$$$\mathrm{cos}\:\mathrm{2x}\:×\:\mathrm{1}\:=\:\mathrm{cos}\:\mathrm{2x}\: \\ $$$$\mathrm{not}\:\mathrm{cos}\:\mathrm{22x}\: \\ $$