Proving a claim regarding differentiability

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SUMMARY

The discussion centers on proving the differentiability of the function f(x,y) under specific conditions related to the function F(x,y,z). It is established that F(x,y,f(x,y)) = 0 near the point M_0(x_0,y_0,z_0), where F has continuous partial derivatives and the gradient at M_0 is non-zero. The conditions imply that f(x,y) cannot be differentiable at (x_0, y_0) due to the zero partial derivative with respect to z at that point. Thus, the conclusion is that f(x,y) is not differentiable at (x_0, y_0).

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gipc
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Let F(x,y,z) be a function which is defined in the point M_0(x_0,y_0,z_0) and around it and the following conditions are satisfied:

1. F(x_0,y_0,z_0)=0
2. F has continuous partial derivatives in M_0 and around it
3. F'_z(x_0,y_0,z_0)=0
4. gradF at (x_0,y_0,z_0) != 0
5. It is known that there is a function f(x,y) so that F(x,y,f(x,y)) =0 in M_0 and around it

Prove that f(x,y) is differentiable in (x_0, y_0)
 
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hmm sorry it seems that I have to prove
that f(x,y) is NOT differentiable in (x_0, y_0)
 

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