sin4a/cos6x=cos2a/sin4a

sin^2(4a)=cos6a*cos2a=0.5(cos8a+cos4a)

пусть 4a=y

2sin^2y=cos2y+cosy

2(1-cos^2y)=cos^2y-sin^2y+cosy

2-2cos^2y-cos^2y+sin^2y-cosy=0

2-3cos^2y+(1-cos^2y)-cosy=0

3-4cos^2y-cosy=0

пусть cosy=t

3-4t^2-t=0

D=1^2-4(-4)*3=49

t1=(1+7)/(-8)=-1; cos4x=-1; 4x=pi+2pil; x=pi/4+pik/2

t2=(1-7)/(-8)=3/4; cos4x=3/4; 4x=+-arcc0s(3/4)+2pik; x=+-0.25 arccos(3/4)+pik/2