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2 cos4x - 2 cos2x = 4 cos^2 x - 1

2 (2 сos²2x-1) - 2cos2x=4 * (1+cos2x) / 2 - 1

4cos²2x-2-2cos2x-2-2cos2x+1=0

4cos²2x-4cos2x-3=0

cos2x=a

4a²-4a-3=0

D=16+48=64

a1 = (4-8) / 8=-1/2⇒cos2x=-1/2

2x=-2π/3+2πn U cos2x=2π/3+2πn

x=-π/3+πn U x=π/3+πn

a2 = (4+8) / 8=1,5⇒cos2x=1,5 нет решения