. COS 5X.COS X COS 4X.COS 2X 3 COS X 1 2 1 ...
3). cos 5x.cos x cos 4x.cos 2x 3 cos x 1
2
1
1 1 cos 4x cos 6x 1 cos 2x cos 6x 3. 1 cos 2x 1
2 2 2
cos 4x cos 6x cos 2x cos 6x 3 1 cos 2x 2
cos 4x 4 cos 2x 5 2 cos 2x 1 4 cos 2x 5
2
(vô nghiệm) hoặc cos 2x 1
cos 2x 2 cos 2x 3
2