y=1/2sin(2X+TT/3)-sinXcosX
=cos(2x+TT/6)sin(TT/6)
=1/2cos(2x+TT/6)
单调递减区间是2kπ≤2x+π/6≤2kπ+π
kπ-π/12≤x≤kπ+5π/12