2cos5x ≤ sqrt(2)
2cos5x ≤ sqrt( 2) cos5x ≤ sqrt( 2)/2 pi/4+2pik ≤ 5x ≤ 7pi/4+2pik, k ∈ z pi/20+2pik/5 ≤ x ≤ 7pi/20+2pik/5, k ∈ z