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__Graphs and Level curves of functions of Two variables__

__Definitions__

- The set of points (x,y) in the plane where a function f (x,y) has a constant value f(x,y)=c is called a
**level curve**of**f** - The set of all points (x,y,f (x,y)) in space, for (x,y) in the domain of
**f,**is called the graph of**f** - The graph of f is called the
**surface**z=f (x,y) - The equation of the following hyperbolic paraboloid is `z=50+x^2-y^2`