Stationary points for surface

[custer]

Find the stationary points for the surface. And find the local and absolute maxima or minima for the following function.
\f(x,y)= xye^{-1/2(x^2+y^2)}

Is there any stationary points for this surface?
Let z = f(x,y)
I found the
\delta{z}/\delta{x} = y[e^{-1/2(x^2+y^2)} - x^2e^{-1/2(x^2+y^2)}]

and

\delta{z}/\delta{x} = x[e^{-1/2(x^2+y^2)} - y^2e^{-1/2(x^2+y^2)}]

I'm supposed to equate the two equations to get the stationary points (x , y, z) But I couldn't since I could only get y - x = y^2 - x^2 from the two equations and that's it.

How do I proceed? Did i do anything wrong?

[tiny-tim]

Hi custer! :smile:

(type \partial not \delta for ∂ :wink:)
Find the stationary points for the surface. And find the local and absolute maxima or minima for the following function.
\f(x,y)= xye^{-1/2(x^2+y^2)}

\delta{z}/\delta{x} = y[e^{-1/2(x^2+y^2)} - x^2e^{-1/2(x^2+y^2)}]

and

\delta{z}/\delta{x} = x[e^{-1/2(x^2+y^2)} - y^2e^{-1/2(x^2+y^2)}]

You got y = x2 and x = y2, and you combined them by adding (or subtracting?) them …

try substituting one into the other instead. :wink:

[custer]

Hi custer! :smile:

(type \partial not \delta for ∂ :wink:)


You got y = x2 and x = y2, and you combined them by adding (or subtracting?) them …

try substituting one into the other instead. :wink:

I think it should be
y[e^{-1/2(x^2+y^2)} - x^2e^{-1/2(x^2+y^2)}]= y(1- x^2) e^{-1/2(x^2+ y^2)}= 0
x[e^{-1/2(x^2+y^2)} - y^2e^{-1/2(x^2+y^2)}]= x(1- y^2)e^{-1/2(x^2+ y^2)}= 0

[tiny-tim]

I think it should be
y[e^{-1/2(x^2+y^2)} - x^2e^{-1/2(x^2+y^2)}]= y(1- x^2) e^{-1/2(x^2+ y^2)}= 0
x[e^{-1/2(x^2+y^2)} - y^2e^{-1/2(x^2+y^2)}]= x(1- y^2)e^{-1/2(x^2+ y^2)}= 0

oops! you're right, I misread it :redface: …

ok … so you have y(1- x2) = 0 and x(1- y2) = 0 …

solve the first equation (there'll be three solutions), and then substitute (each solution into the second equation). :smile: