cosx≥√3/2=cos(±π/6)
cosx在[-π,0]为增区间,[0,π]为减区间
cosx≥√3/2则-π/6≤x≤π/6
cosx是周期函数 T=2π
{X|-π/6+2Kπ≤x≤π/6+2Kπ,K∈N}