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]