- #1
Dansuer
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Homework Statement
Discuss the continuity, derivability and differentiability of the function
[itex]f(x,y) = \frac{x^3}{x^2+y^2}[/itex] if (x,y)≠(0,0) and 0 otherwise
Homework Equations
if f is differentiable then [itex]∇f.v=\frac{∂f}{∂v}[/itex]
if f has both continuous partial derivative in a neighbourhood of [itex]x_0[/itex] then it's differentiable in [itex]x_0[/itex]
The Attempt at a Solution
I have no problem with continuity.
For derivability i consider the definition of directional derivative in an arbitray direction (α,β)
[itex]\frac{f(αt,βt)-f(0,0)}{t}=\frac{α^3t^3-0}{t^3}=α^3[/itex]
all the directional derivatives exist.
this equation [itex]∇f.v=\frac{∂f}{∂v}[/itex] tells me that the directional derivative should depend linearly on α and β which is not the case, f is not differentialble in [itex]x_0[/itex]
on the other hand it's easy to calculate the partial derivatives in a neighborood of (0,0) and see that they are continous.
[itex]f(x,0)=x \ \ \ f(0,y)=0[/itex]
[itex]\frac{∂}{∂x}f(x,0)=1 \ \ \ \frac{∂}{∂y}f(0,y)=0[/itex]
i get two different results with two different approaches and i can't figure out what's wrong with one of them, or both :tongue2:
any help is appreciated