Вычислите: cos8/15П cos1/5П + sin8/15П sin 1/5П

Question

Вычислите:
cos8/15П cos1/5П + sin8/15П sin 1/5П

Answer 1

Cos(a+b)= cosacosb - sinasinb
cos(a-b) = cosacosb + sinasinb
cos(8pí/15-pí/5) =cos(5pí/15) =cos(pí/3)=1/2