2kπ+π/2＜a＜2kπ+π
kπ+π/4＜a/2＜kπ+π/2
kπ-π/2＜-a/2＜kπ-π/4
kπ+π/2＜π-a/2＜kπ+3π/4
故π-α/2是第一、四象限角
2kπ+0＜a＜2kπ+π/2
2kπ/3＜a/3＜2kπ/3+π/6
故α是第一、二象限角
若α是第二象限角,判断π-α/2是第几象限角
2kπ+π/2＜a＜2kπ+π
kπ+π/4＜a/2＜kπ+π/2
kπ-π/2＜-a/2＜kπ-π/4
kπ+π/2＜π-a/2＜kπ+3π/4
故π-α/2是第一、四象限角
2kπ+0＜a＜2kπ+π/2
2kπ/3＜a/3＜2kπ/3+π/6
故α是第一、二象限角