Which of the following is not a Linear Differential Equation?

$\displaystyle \left(1+\mathrm{x}^{2}\right) \mathrm{d} \mathrm{y}+2 \mathrm{x} \mathrm{y} \mathrm{d} \mathrm{x}=\cot \mathrm{x} \mathrm{d} \mathrm{x}$

$\displaystyle \mathrm{y}+\frac{\mathrm{d}}{\mathrm{dx}}(\mathrm{xy})=\mathrm{x}(\sin \mathrm{x}+\log \mathrm{x})$

$\displaystyle \mathrm{x}\left(1+\mathrm{y}^{2}\right) \mathrm{dx}-\mathrm{y}\left(1+\mathrm{x}^{2}\right) \mathrm{dy}=0$

$\displaystyle \mathrm{ydx}-\left(\mathrm{x}+3 \mathrm{y}^{2}\right) \mathrm{dy}=0$