Tangent plane of a parametric function

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SUMMARY

The discussion focuses on finding the equation of a tangent plane for a parametric function represented as ##r(u,v)=f(u,v)\hat{i} + g(u,v)\hat{j} +h(u,v)\hat{k}## at a specific point ##(u_{0},v_{0})##. The key method involves calculating the cross product of the partial derivatives ##\frac{\partial r}{\partial u}## and ##\frac{\partial r}{\partial v}## to obtain the normal vector to the surface. The user also contemplates the implications of adding a third parameter, ##w##, but concludes that for surfaces, only the two-parameter case applies. This confirms that the normal vector can be consistently derived from the cross product of the two relevant partial derivatives.

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BiGyElLoWhAt
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Ok, so I'm really hoping someone can help me logic my way through this.

I have a function to the effect of: ##r(u,v)=f(u,v)\hat{i} + g(u,v)\hat{j} +h(u,v)\hat{k}##

I need to find an equation of a tangent plane at a point ##(u_{0},v_{0})##
and quite frankly I'm at a loss on how to do this.

So what I ultimately want is 2 vectors tangent to the surface at that point, I'm not sure that with what I have I can simply chain some derivatives together and cross them...

If I had z(x,y) I would simply take ##\frac{\partial z}{\partial x} (cross) \frac{\partial z}{\partial y}## as that would give me 2 (presumably different) vectors and crossing them will give me my normal to use in my tangent function...

Now how to apply this to a parametric function in 3-space...

Ok... I think I just answered my question as I was typing this... (It happens that way quite a bit)
##\frac{\partial r}{\partial u} (cross) \frac{\partial r}{\partial v}##

But what if I had ##r(u,v,w)## ?
In my head , whether I chose to cross
##\frac{\partial r}{\partial u} (cross) \frac{\partial r}{\partial v}##
or
##\frac{\partial r}{\partial u} (cross) \frac{\partial r}{\partial w}##
or
##\frac{\partial r}{\partial w} (cross) \frac{\partial r}{\partial v}##
It really shouldn't matter, I guess at this point I just want to hear somebody's input other than my own. All of these will give me a normal, no?

(once I plug in my point that is...)
 
Last edited:
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A surface only has two parameters. So you will only run into the ##r_u\times r_v## case for a surface. And you are correct, that is how you find a normal.
 

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