-根号2/2 ≤ cos x ≤ 1/2
2kπ+π/3 ≤ x ≤ 2kπ+3π/4,2kπ-3π/4 ≤ x ≤ 2kπ-π/3,其中k∈Z
即：
x∈【2kπ-3π/4,2kπ-π/3】,【2kπ+π/3 ,2kπ+3π/4】k∈Z