4(1-cos4x)²/4+3cos4x-1=0
1-2cos4x+cos²4x+3cos4x-1=0
cos²4x+cos4x=0
cos4x*(cos4x+1)=0
cos4x=0⇒4x=π/2+πn⇒x=π/8+πn/2
π≤π/8+πn/2≤3π/2
8≤1+4n≤12
7≤4n≤11
7/4≤n≤11/4
n=2⇒x=π/8+π=9π/8
cos4x=-1⇒4x=π+2πk⇒x=π/4+πk/2,k∈z
π≤π/4+πk/2≤3π/2
4≤1+2k≤6
3≤2k≤5
3/2≤k≤5/2
k=2⇒x=π/4+π=5π/4