Proof of stationary points of 3D function

In summary, to show that a function f(x,y) is stationary, we can show that df/dx=0 and df/dy=0 (the d's being partial derivatives). This can be achieved by setting g=0 and h=0, or by using the product rule for partial derivatives and showing that df/dx=0 and (dg/dx)(dh/dy)=(dg/dy)(dh/dx).
  • #1
Lucy Yeats
117
0

Homework Statement



Show that f(x,y)=g(x,y)h(x,y) is stationary if and only if:
g=0 and h=0
OR
df/dx=0 and (dg/dx)(dh/dy)=(dg/dy)(dh/dx)
(All the d's in the line above should be curly d's for partial derivatives.)

Homework Equations





The Attempt at a Solution



I tried expressing df as a total derivative then setting this equal to zero, but I wasn't getting anywhere.
 
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  • #2
Any suggestions would be great. :)
 
  • #3
f is stationary if df/dx=0 and df/dy=0 (the d's being partial derivatives). g=0 and h=0 are certainly sufficient to show that. Just use the product rule. Where did you go from there? Can you show your work? Use that one of g or h must be nonzero.
 
Last edited:

Related to Proof of stationary points of 3D function

1. What is a stationary point in a 3D function?

A stationary point in a 3D function is a point where the gradient (or slope) of the function is equal to zero. This means that at a stationary point, the function is neither increasing nor decreasing in any direction.

2. How do you determine if a point is a stationary point in a 3D function?

To determine if a point is a stationary point in a 3D function, you must calculate the partial derivatives of the function with respect to each variable. If both partial derivatives are equal to zero at a specific point, then that point is a stationary point.

3. What is the significance of stationary points in 3D functions?

Stationary points in 3D functions can be important in understanding the behavior of the function. They can indicate the location of maximum or minimum values, as well as points of inflection. They can also be useful in optimization problems.

4. How can you determine the nature of a stationary point in a 3D function?

The nature of a stationary point in a 3D function can be determined by analyzing the second derivative at that point. If the second derivative is positive, the point is a local minimum. If the second derivative is negative, the point is a local maximum. And if the second derivative is zero, further analysis is needed to determine the nature of the point.

5. Can a 3D function have more than one stationary point?

Yes, a 3D function can have multiple stationary points. These points can be local or global maxima or minima, or points of inflection. The number and location of stationary points in a 3D function can vary depending on the complexity of the function.

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