Use the Chain Rule to find the indicated partial derivatives.
$u = \sqrt{r^2 + s^2}$, $r = y + x \cos(t)$, $s = x + y \sin(t)$;
$\frac{\partial u}{\partial x}$, $\frac{\partial u}{\partial y}$, $\frac{\partial u}{\partial t}$ when $x = 1$, $y = 2$, $t = 0$
$\frac{\partial u}{\partial x} = \boxed{}
$\frac{\partial u}{\partial y} = \boxed{}
$\frac{\partial u}{\partial t} = \boxed{}$
