Cos2x(2cos²2x -1) =1/4 ;
2cos³2x -cos2x -1/4 =0 ;
2(cos2x+1/2)*(cos²2x -1/2cos2x -1/4) =0 ;
a) cos2x+1/2 =0 ;
cos2x = -1/2;
x= (+/-)π/3+πk , k∈Z .
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b) cos²2x - 1/2cos2x -1/4 =0 ;
cos2x =(1+√5)/4 ;
2x =(+/-)arccos(1+√5)/4 +2π*k ;
x = (+/-) (1/2)*arccos(1+√5)/4 +π*k ;
cos2x =(1-√5)/4 ;
2x =(+/-)arccos(1-√5)/4 +2π*k ;
x = (+/-) (1/2)*arccos(1-√5)/4 +π*k ;