Đáp án:

• \( I = \int \frac{2 \sin x .\cos^2 x}{1 + \cos x} \, dx \quad \)Đặt \( t = \cos x \)

\( = -2 \int \frac{t^2}{t + 1} \, dt = -2 \int \left( t - 1 + \frac{1}{t + 1} \right) \, dt \)

\( = -2 \left( \frac{t^2}{2} - t + \ln |t + 1| \right) + C \)