5arcsim√3/2 4arcsin1/2-3arctg (-1/√3)"

5arcsin√3/2=5*π/3=5π/3

4arcsin1/2=4*π/6=2π/3

3arctg (-1/√3) = 3 * (-π/6) = - π/3

5arcsin√3/2+4arcsin1/2-3arctg (-1/√3) = 5π/3+2π/3+π/3=8π/3

cos (x-π/5) ≤√2/2

π/4+2πn≤x-π/5≤7π/4+2πn

9π/20+2πn≤x≤39π/20+2πn

x∈[9π/20+2πn; 39π/20+2πn]