A \( y_2 + 9y_1 = 3 \cos 3x \)
B \( y_2 - 9y_1 = 3 \cos 3x \)
C \( y_2 + 9y = \frac{3}{2} \cos 3x \)
D \( y_2 + 9y = 3 \cos 3x \)
E \( y_2 + 9y = 3 \sin 3x \)
Correct Answer: Option D
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