a+β=-2(cosθ+1)
aβ=cos^2(θ)
α-β的绝对值≤2√2,
(a-β)^2≤8
a^2-2aβ+β^2≤8
(a+β)^2-4aβ≤8
4(cosθ+1)^2-4cos^2(θ)≤8
2cosθ+1≤2
cosθ≤1/2
2kπ+π/3≤θ≤2kπ+5π/3