4sin^4х+7 cos2x=1 1/2cos4x - 2cos2x +3/2 +7cos2x-1=0 cos^2 2x -1/2 + 1/2 + 5cos2x =0 cos^2 2x + 5cos2x =0 cos2x(cos2x+5)=0 1)cos2x+5=0 cos2x=-5 пустое ножество 2) cos2x=0 2x=pi/2 + pi n x=pi/4 + pi*n/2 n принадлежиит z