(π/4) + 2πn ≤ x-(π/5) ≤ (3π/4)+2πn, n ∈ Z
Прибавляем ко всем частям неравенства (π/5) :
(π/4)+(π/5) + 2πn ≤ x ≤ (3π/4)+(π/5)+2πn, n ∈ Z
(9π/20) + 2πn ≤ x ≤ (19π/20)+2πn, n ∈ Z
2)
-cos 8x >-0,5 ⇒
cos 8x < 0,5
(–7π/6)+2πk < 8x < ( π/6)+2πk, k ∈ Z
Делим на 8
(–7π/48)+(π/4)*k < x < ( π/48)+(π/4)*k, k ∈ Z