y=log1/2x 递减
sin{((2π)/3)-2x}=sin{π-(2π/3-2x)}=sin(2x+π/3)递增且大于零
2kπ＜2x+π/3≤2kπ+π/2
2kπ-π/3＜2x≤2kπ+π/2-π/3
kπ-π/6＜x≤kπ+π/12
-π/6＜x≤π/122kπ＜2x π/3≤2kπ π/2为啥要这样 x 前的符号为啥变正