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The number of solutions of the equation x 2 cos 4 1 x 3 cos 3 x cos 2 = ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − π ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ π + , x ∈ [–3 π, 3 π ] is

1. 8
2. 5
3. 6
4. 7

Correct answer: 7

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