Question

Люди плизззззззззззз ало

Answer 1

Ответ:

$cos21^\circ \, cos9^\circ - cos36^\circ \, cos24^\circ =\\\\\\=\dfrac{1}{2}\cdot (cos30^\circ +cos12^\circ )-\dfrac{1}{2}\cdot (cos60^\circ +cos12^\circ )=\\\\\\=\dfrac{1}{2}\cdot \Big(\dfrac{\sqrt3}{2}+cos12^\circ \Big)-\dfrac{1}{2}\cdot \Big(\dfrac{1}{2}+cos12^\circ \Big)=\dfrac{\sqrt3}{4}+\dfrac{1}{2}\, cos12^\circ -\dfrac{1}{4}-\dfrac{1}{2}\, cos12^\circ =\\\\\\=\dfrac{\sqrt3}{4}-\dfrac{1}{4}=\dfrac{\sqrt3-1}{4}$