Надо! cos π/8 cosx-sin π/8 sinx ≤ -√3/2

cos π/8 cosx-sin π/8 sinx ≤ -√3/2
cos(π/8+x)≤-√3/2
5π/6+2πn≤π/8+x≤7π/6+2πn, n∈Z
5π/6-π/8+2πn≤x≤7π/6-π/8+2πn, n∈Z
17π/24+2πn≤x≤25π/24+2πn, n∈Z
x∈[17π/24+2πn; 25π/24+2πn], n∈Z