9-x^2≥0且1-2cosx＞0
解9-x^2≥0得-3≤x≤3
1-2cosx＞0得x＞2kπ+π/3或x＜2kπ-π/3
而-3≤x≤3,所以只能x＞π/3或x＜-π/3
所以定义域为D={x|-3≤x＜-π/3或者π/3＜x≤3}