Question Number 192624 by sciencestudentW last updated on 23/May/23
$${cos}\mathrm{36}−{cos}\mathrm{72}=? \\ $$
Answered by som(math1967) last updated on 23/May/23
$$\mathrm{2}{sin}\mathrm{54}{sin}\mathrm{18} \\ $$$$=\mathrm{2}×\frac{\sqrt{\mathrm{5}}+\mathrm{1}}{\mathrm{4}}×\frac{\sqrt{\mathrm{5}}−\mathrm{1}}{\mathrm{4}} \\ $$$$=\mathrm{2}×\frac{\mathrm{5}−\mathrm{1}}{\mathrm{16}}=\frac{\mathrm{1}}{\mathrm{2}} \\ $$