y=log1/2cos(x/3+π/4)
t=cos(x/3+π/4)
y=log1/2(t)在定义域内是减函数
要使y=log1/2cos(x/3+π/4)是减函数
t=cos(x/3+π/4)必须是增函数
x/3+π/4在[2kπ-π,2kπ]增函数
x在[6kπ-15π/4,6kπ-3π/4]增函数cos(x/3+π/4)好像只能大于0吧 那应该是x/3+π/4在（2kπ-π/2,2kπ]增函数吧应该是x/3+π/4在（2kπ-π/2,2kπ]增函数但cos(x/3+π/4)只能大于0所以：x/3+π/4在[2kπ-π/2,2kπ+π/2]正确：（ -9π/4+6kπ，-3π/4+6kπ ] k属于z