On Problems $(9)$ through $(11),$ first verify that $y(x)$ satisfies the given differential equation. Then determine a value of the constant $C$ so that $y(x)$ satisfies the given initial condition.

$9.y^{\text{'}}+y=0$;$y=C{e}^{-x},y(0)=8$

$10.y^{\text{'}}-1=y$;$y=C{e}^x-1,y(0)=2$

$11.e^y{y}^{\text{'}}=1$;$y=ln(x+C),y(0)=1$