Why is it obvious that the plane $z=0$ is tangent to the surface $z=x^{2}+y^{2}$ Why is it obvious that the plane $z=0$ is tangent to the surface $z=x^{2}+y^{2}$
I don't quite understand, is this obvious? I have a problem with the background knowledge, I don't even know how to deal with the surface $z=x^{2}+y^{2}$.
 A: Define the surface $\;f(x,y,z)=x^2+y^2-z=0\;$ , so that
$$\frac{\partial f}{\partial x}=2x\,,\;\;\frac{\partial f}{\partial y}=2y\,,\;\;\frac{\partial f}{\partial z}=-1$$
Then the tangent plane to $\;f\;$ at $\;(0,0)\;$ is given by
$$\frac{\partial f(0,0)}{\partial x}(x-0)+\frac{\partial f(0,0)}{\partial y}(y-0)+\frac{\partial f(0,0)}{\partial z}(z-0)=0\implies-z=0$$
A: An explanation is that for an algebraic surface with equation  $p(x,y,z)=0$, $p$ being a polynomial such that $p(0,0,0)=0$, the tangent plane at the origin has as an equation the linear approximation of $p(x,y,z)$ in a neighbourhood of $(0,0,0)$, i.e. the linear part of the polynomial $p(x,y,z)$.
A: If you want to imagine why this is true, think about how $x^2 + y^2 = z$ changes as $z$ changes by looking at the level curves.  The curve $x^2 + y^2 = 1$, for example, is a circle of radius 1, and $x^2 + y^2 = 0$ is a point.  So, as $z$ grows, the radius of the circle grows, and the surface looks like a big circular bowl sitting on the $xy$-plane (aka $z=0$).  The surface just meets the plane at the point $(0,0)$.  
A: The surface is a parabolic bowl, sitting on the plane $z=0$, with the $z$ axis as its axis of symmetry. A typical slice of it, cut perpendicular to its axis, is a circle centred on the $z$ axis of radius $\surd z$. Cutting it another way—say with the $y=0$ plane—gives a parabola $x^2=z$ in the plane, and similarly with any other plane through the $z$ axis.
A: In fact at $z=0$ the equation $x^2+y^2=0$ defines only $1$ point, which is $(0,0)$.
So the plane $z=0$ and the surface intersect in a single point. 
But since the surface is smooth (the equation is polynomial), it cannot have singular points, and the touching point is automatically a tangent point.
