cos7xcos3x-sin3xcos7x <-1/2

Cos7xcos3x-sin3xcos7x <-1/2
cosαcosβ-sinαcosβ=cos(α+β)
Тогда   сos7xcos3x-sin3xcos7x=cos(7x+3x)+cos10x <-1/2 
2π/3+2πn<10x<7π/6+2πn,2π/10·3+2πn/10<x<7π/10·6+2πn/10,
π/15+πn/5<x<7π/60+πn/5,n∈Z