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## Homework Statement

given f(x,y)=9/(x+y) find a level surface.

## Homework Equations

## The Attempt at a Solution

g(x,y,z)=f(x,y)-z=0?

g(x,y,z)=9/(x+y) -z=0?

That answer is wrong. Apparently i must have the following:

g(x,y,z)=z(x+y)=9

How do I solve problems like these?

Another example is to find a function f(x,y,z) whose level surface f=5 is the graph of the paraboloid

g(x,y)=-x^2 -y^2