1. Parametrization of Hyperboloid

a) Find a parametrization of the hyperboloid $x^2 + y^2 - z^2 = 25$
b) Find an expression for a unit normal to this surface.
c) Find an equation for the plane tangent to the surface at $(x_0,y_0,0)$, where $x_0^2 + y_0^2 = 25$

For the parametrization I got $\Phi(r,\theta) = (rcos\theta, rsin\theta, \sqrt{r^2 - 25})$

is that the correct parametrization?

2. Let $x = rcos(\theta)$ and $y = rsin(\theta)$.

3. Think about an auxilliary plane uz, for example, which is perpendicular to the $xy$ plane and makes $\theta$ degress starting from x axis. In this plane, we will make each point $P = ( Pu, Pz )$ rotate. But $Pu$ and $Pz$ will have a parametrization too thus this will nicely describe the hyperboloid. Lets see:

In the $uz$ plane, you parametrize the hyperbola with $Pu = cosh (s)$ and $Pz = sinh (s)$ with $-1\leq s \leq 1$. Notice that this parameter s will describe the contour of the hyperbola.

Now you should use $\theta$ as another parameter, an this would be the "revolution parameter". It will take each contour of hyperbola and rotate it $\theta$ degrees.

So, in the $xy$ plane, we should have $x = 5Pu\cos\theta$ and $y=5Pu\sin\theta$ with $0\leq \theta \leq 2\pi$. If we make $z = 0$ in $x^2 + y^2 - z^2 = 25$, we would end with $x^2 + y^2 = 25$, which requires the $r = 5$ in the parametrization $x = r\cos\theta$ and $y = r\sin\theta$.

Putting all together, we must have: $\Phi(s,\theta) = ( Pu\cos\theta, Pu\sin\theta, Pz) = ( 5\cosh(s)\cos\theta, 5\cosh(s)\sin\theta, 5sinh (s) )$

For the B) you want the expression of the normal vector?

4. Only to ellucidate, here's the hyperboloid. We can see the mesh caused by this type of parametrization - there are circunferences parallel to the xy plane, in which we have some z coordinate fixed, and there are hyperbolas, when we have either x or y fixed.

For the B, I guess you should use the definition of gradient. I'll work on this later. I hope it helps!

5. Originally Posted by larryboi7
a) Find a parametrization of the hyperboloid $x^2 + y^2 - z^2 = 25$
b) Find an expression for a unit normal to this surface.
c) Find an equation for the plane tangent to the surface at $(x_0,y_0,0)$, where $x_0^2 + y_0^2 = 25$

For the parametrization I got $\Phi(r,\theta) = (rcos\theta, rsin\theta, \sqrt{r^2 - 25})$

is that the correct parametrization?
Yes, if $x= r cos(\theta)$, $y= r sin(\theta)$, and $z= \sqrt{r^2- 25}$, then $x^2+ y^2- z^2= r^2 sin^2(\theta)+ r^2 cos^2(\theta)- (r^2- 25)= r^- (r^2- 25)= 25$
(Although there is no such thing as "the" parameterization. A surface has many different parameterizations. Since your x, y, and z satisfy the equation $x^2+ y^2- z^2= 25$, it is a valid parameterization.)

To find a vector perpendicular to the surface $x^2+ y^2- z^2= 25$ think of that as a "level surface" of $F(x,y,z)= x^2+ y^2- z^2$ and use the fact that grad F is always perpendicular to level surfaces of F.

And, once you have a perpendicular, use the fact that if the vector $A\vec{i}+ B\vec{j}+ C\vec{k}$ at point $(x_0,y_0,z_0)$ in the plane, then the plane is given by $A(x- x_0)+ B(y- y_0)+ C(z- z_0)= 0$.