Find the particular solution of the following differential equations:
\(\frac{y}{x} \frac{\mathrm{d}y}{\mathrm{d}x} = \frac{y^2 + 1}{x^2 + 1}\); \(y = 0\) when \(x = 1\)
\(e^t \frac{\mathrm{d}s}{\mathrm{d}t} = \sqrt{s}\); \(s = 4\) when \(t = 0\)
\(x^2 \frac{\mathrm{d}y}{\mathrm{d}x} + 1 + y^2 = 0\); \(y = 1\) when \(x = \frac{4}{\pi}\)
\((1 + \cos 2x) \frac{\mathrm{d}y}{\mathrm{d}x} = y \sin 2x\); \(y = 2\) when \(x = \frac{\pi}{4}\)