Cos 5п/9 cos 13п/9 - sin 5п/9 sin 13п/9=

Question

Cos 5п/9 cos 13п/9 - sin 5п/9 sin 13п/9=

Answer 1

Решение
Cos 5п/9 cos 13п/9 - sin 5п/9 sin 13п/9= cos(5π/9 + 13π/9) = 
cos(18π/9) = cos2π = 1