cosx的单调递增区间∈（2kπ-π,2kπ）k∈Z
-1/2 ≤ cosx ＜根号3/2
cos(-2π/3)=-1/2
cos（-π/6)=根号3/2
∴2kπ-2π/3 ≤ x ＜ 2kπ-π/6 k∈Z