$\int \cos^2 3x \, dx = কত?$ Chittagong • 2023 ক (ক) $\frac{1}{2} (x + \sin 6x) + c$ খ (খ) $\frac{1}{2} (x - \sin 6x) + c$ গ (গ) $\frac{1}{12} (6x + \sin 6x) + c$ ঘ (ঘ) $\frac{1}{12} (6x - \sin 6x) + c$