Đáp án:  

•  \( \cos 3x = 4\cos^3 x - 3\cos x \implies \cos^3 x = \frac{1}{4}\left[\cos 3x + 3\cos x\right] \)

    \( \int \cos^3 x \, dx = \frac{1}{4} \int \left(\cos 3x + 3\cos x\right) dx \)

    \( = \frac{1}{4} \left[\frac{1}{3} \sin 3x + 3\sin x \right] + C \)

Cách 2:
            \( \int \cos^3 x \, dx = \int \cos x \left(1 - \sin^2 x\right) dx \)

                                   \( = \int \cos x \, dx - \int \cos x \sin^2 x \, dx = \sin x - \frac{\sin^3 x}{3} + C \)