Identify the DE that is not linear.

y$\text{'}\text{'}+$ cos y $=$ x

$(2$x$)$ y$\text{'}\text{'}\text{'}+(1/$ x$)$ y $=0$

x$5$ y$\text{'}\text{'}+$ x y$\text{'}+$ x$2$ y $=14$

$($x $+1)$ y$\text{'}\text{'}+($sin x$)$ y$\text{'}+($cos x$)$ y $=$ ex