Concept: First-order linear differential equation.
Step 1: \[ \frac{dy}{dx} - \frac{\tan 2x}{\cos^2 x}\, y = \cos^2 x \]
Step 2: \[ \tan 2x = \frac{2\tan x}{1 - \tan^2 x}, \quad \cos^2 x = \frac{1}{1 + \tan^2 x} \]
Step 3: \[ y = \frac{1}{2} \cdot \frac{\sin 2x}{1 - \tan^2 x} \]
Step 4:
\( x = \frac{\pi}{6} \) satisfies the given condition.