Trả lời chủ đề

pt <=>(3sinx-4sin^3 x)sin^3 x+(4cos^3 x-3cosx)cos^3 x=-1

   <=> 3sin^4 x -4sin^6 x+4cos^6 x-3cos^4 x=-1

   <=> 3(sin^2 x-cos^2 x)-4(sin^2 x-cos^2 x)(1+sin^2 xcos^2 x)=-1

   <=> (sin^2 x-cos^2 x)(3-4-4sin^2 xcos^2 x)=-1

   <=> (sin^2 x-cos^2 x)(-1+sin^2 x)=-1

   <=> cos 2x.cos^2 2x=-1

   <=> cos^3 2x=-1