How do I find the equation of the normal line for a given function and point?

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SUMMARY

The discussion focuses on finding the equation of the normal line for the function f(x,y) = x² + y² - 1 at the point (2, 1, 4). The normal vector is derived from the gradient of the level surface g(x, y, z) = x² + y² - z, resulting in ∇g = 2x i + 2y j - k. Specifically, at the point (2, 1, 4), the normal vector is 2 i + 4 j - k. The solution involves deriving parametric equations for the normal line using this direction vector.

PREREQUISITES
  • Understanding of gradient vectors and their geometric interpretation
  • Familiarity with parametric equations of lines in three-dimensional space
  • Knowledge of level surfaces and their properties
  • Basic calculus concepts, particularly partial derivatives
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  • Study the properties of gradient vectors and their applications in multivariable calculus
  • Learn how to derive parametric equations from a point and a direction vector
  • Explore the concept of level surfaces in three-dimensional space
  • Review examples of normal lines to surfaces in calculus
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Students studying multivariable calculus, particularly those learning about normal lines and gradients, as well as educators looking for examples to illustrate these concepts.

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Homework Statement


Find the equation of the normal line of function f(x,y) = x^2+y^2-1 through points (2,1,4)


Homework Equations


(x-xo)/f_x = (y-yo)/f_y = (z-zo)/f_z


The Attempt at a Solution


so...
f_x = 2x = 4
f_y = 2y = 2
f_z = ?

I don't know how to solve for f_z...
here is random stuff I was doing that *might* be related?
f(x,y) - z = 0
f(x,y) = z = 4

yeah.. any math guru want to show this grasshopper the way?
 
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If z = f(x,y) then z -1 = ##x^2 + y^2##. What kind of surface is this? Based on elementary geometry you should know what the normal line looks like. And if you get that far, you can solve this without any calculus at all. I recommend you do so.

That is probably not what your teacher had in mind. However, there is nothing like knowing the answer to help you solve the problem. (I'm not actually joking here -- if you have no idea what the answer should look like you are not ready to tackle a solution).

Given that I've told you how to express z, and provided a hint about what this line should look like, can you proceed now?
 
You don't "solve for f_z". Your "f" is NOT a function of z!

z= f(x,y)= x^2+ y^2- 1 is the same as x^2+ y^2- z= 1. We can think of that as a "level surface" for function g(x, y, z)= x^2+ y^2- z. Now, g is a function of x, y, and z with g_x= 2x, g_y= 2y[/tex], and g_z= -1. So \nabla g= 2x\vec{i}+ 2y\vec{j}- \vec{k} is perpendicular to that surface at every (x, y, z). In particular, at (2, 1, 4) this is 2\vec{i}+ 4\vec{j}- \vec{k}.<br /> <br /> So the problem of &quot;find the normal line&quot; reduces to &quot;find parametric equations of the line through (2, 1, 4) with direction given by 2\vec{i}+ 4\vec{j}- \vec{k}&quot;. Can you do that?
 

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