Sin^2 - cos^2 = cos4x

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6.
7.
8.
sin² x - cos²x = cos 4x.
cos 3x - cos 5x = sin x.
3sin² x+4 cos x - 2 = 0.
sin² x + sin² 2x = sin² 3x.
9.
10. 2tanx+ 3cot x = 4.
11. 2 cos² x - 3sin 2x + sin²x = 1.
12. 2 sin² x + sin x cos x - cos²x = 3.
13. 3 sinx - 4 cos x = 1.