For instance, you have the circle:
$$ x^2 + y^2 = 1$$ It is differentiable on both $x$ and $y$ and it describes a bounded set.
Now you could pick an easier equation, like $x+y=1$, which is linear instead of quadratic. However it does not describe a bounded set...
It has to be only one an differentiable equation, so stuff like $||(x,y)||_\infty = 1$ or $\{x+y=1, x \geq 0, y \geq 0\}$ don't count. So what are some easy equations differentiable on both $x$ and $y$ that describe a bounded set? Easy as in involving only simple functions of $x$ and $y$. I'd rather they be about as easy as the actual circle, or preferably easier