|cos(π/2-a)|=cos(π/2+a)
|cos(π/2)cosa+sin(π/2)sina|=cos(π/2)cosa-sin(π/2)sina
|sina|=-sina
sina≤0
2kπ+π≤a≤2kπ+2π(k为整数)
角a的取值集合[2kπ+π,2kπ+2π](k为整数)