Đáp án:

• \( \int \frac{\sin^2 x}{\cos^4 x} \, dx = \int \frac{1}{\cos^2 x} \cdot \tan^2 x \, dx = \frac{\tan^3 x}{3} + C \)

• Cách 2:

• \( I = \int \frac{1 - \cos^2 x}{\cos^4 x} \, dx = \int \frac{1}{\cos^4 x} \, dx - \int \frac{1}{\cos^2 x} \, dx \)