α∈II
所以α∈(2kπ+π/2,2kπ+π),k∈Z
α/2∈(kπ+π/4,kπ+π/2),k∈Z
如果k是偶数,设k=2m,m∈Z,则α/2∈(2mπ+π/4,2mπ+π/2),k∈Z,在I
如果k是奇数,设k2m+1,m∈Z,则α/2∈(2mπ+5π/4,2mπ+3π/2),k∈Z,在III