π+2kπ＜a＜3π/2+2kπ
-π+2kπ＜-a＜-3π/2+2kπ.都+2π,得π/2+2kπ＜-a＜π+2kπ
2π+2kπ＜2a＜3π+2kπ,都-2π,得2kπ＜2a＜π+2kπ
π/3+2kπ＜a/3＜π/2+2kπ
如果角α是第三象限角,分别求-α,2α,α/3的范围
π+2kπ＜a＜3π/2+2kπ
-π+2kπ＜-a＜-3π/2+2kπ.都+2π,得π/2+2kπ＜-a＜π+2kπ
2π+2kπ＜2a＜3π+2kπ,都-2π,得2kπ＜2a＜π+2kπ
π/3+2kπ＜a/3＜π/2+2kπ