amaths

2sinxsin3x=1
-cos(x+3x)+cos(x-3x)=1
-cos4x+cos2x=1
-2cos²2x+1+cos2x=1
cos2x(1-2cos2x)=0
cos2x=0 or cos2x=1/2
2x=2nπ±π/4 or 2x=2nπ±π/3
x=nπ±π/8 or x=nπ±π/6