sin(2x-3π/4)>=-1/2
所以-π/6+2kπ≤2x-3π/4≤7π/6+2kπ,
所以7π/12+2kπ≤2x≤23π/12+2kπ
解得7π/24+kπ≤x≥23π/24+kπ,k是整数.