Question Number 107976 by anonymous last updated on 13/Aug/20
$${Rewrite}\:\mathrm{cos6}{x}\mathrm{cos}\:\mathrm{4}{x}\:{as}\:{a}\:{sum}\:{or} \\ $$$${difference} \\ $$
Answered by mathmax by abdo last updated on 13/Aug/20
$$\mathrm{cos}\left(\mathrm{4x}\right)\mathrm{cos}\left(\mathrm{6x}\right)\:=\frac{\mathrm{1}}{\mathrm{2}}\left\{\mathrm{cos}\left(\mathrm{10x}\right)\:+\mathrm{cos}\left(\mathrm{2x}\right)\right\} \\ $$