For the exact differential equation,

$\frac{du}{dx}=\frac{-x{u}^2}{2+x^{2}u}$

which one of the following is the solution?

Options :

1. u² + 2x² = constant

2. xu² + u = constant

3. $\frac{1}{2}{x}^2{u}^2+2u$ = constant

4. $\frac{1}{2}u{x}^2+2u$ = constant