Question

И вот одно плиз
cos45°cos15°=

Answer 1

$cos45^{\circ}cos15^{\circ}= \frac{cos(45-15)+cos(45+15)}{2} = \frac{cos(30^{\circ})+cos(60^{\circ})}{2} = \frac{ \frac{\sqrt{3}}{2} + \frac{1}{2} }{2}=\\= \frac{ \frac{\sqrt{3}+1}{2} }{2} = \frac{\sqrt{3}+1}{4}$