Proof of stationary points of 3D function

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SUMMARY

The function f(x,y) = g(x,y)h(x,y) is stationary if and only if either g = 0 and h = 0, or the partial derivatives satisfy df/dx = 0 and (dg/dx)(dh/dy) = (dg/dy)(dh/dx). The discussion emphasizes the importance of using the product rule for derivatives to establish these conditions. Participants noted that one of g or h must be non-zero to apply the necessary conditions for stationarity effectively.

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  • Understanding of partial derivatives and notation (curly d's)
  • Familiarity with the product rule in calculus
  • Basic knowledge of stationary points in multivariable functions
  • Experience with functions of multiple variables
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Lucy Yeats
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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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Any suggestions would be great. :)
 
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.
 
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