Sin()*cos()
1/2sin(2π/3-4x)≥-√3/4 sin(2π/3-4x)≥-√3/2 sin(4x-2π/3)≤√3/2 2π/3+2πn≤4x-2π/3≤7π/3+2πn 4π/3+2πn≤4x≤3π+2πn π/3+πn/2≤x≤3π/4+πn/2,n∈z