- #1
Chromosom
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Homework Statement
Prove that function has directional derivative in every direction, but is not differentiable in (0,0):
[tex]f(x,y)=\begin{cases}\frac{x^3}{x^2+y^2},&(x,y)\neq(0,0)\\ \\0,&(x,y)=(0,0)\end{cases}[/tex]
The Attempt at a Solution
I have already proved that it has directional derivative in every direction. But in my opinion, if it has directional derivative in direction of OX and OY, it has partial derivatives, so that it is differentiable. Let's derive it:
[tex]\frac{\partial f}{\partial x}=\lim_{h\to0}\frac{f(h,0)-f(0,0)}{h}=\lim_{h\to0}\frac{\frac{h^3}{h^2+0}}{h}=\lim_{h\to0}\frac{h^3}{h^3}=1[/tex]
[tex]\frac{\partial f}{\partial y}=\lim_{h\to0}\frac{f(0,h)-f(0,0)}{h}=\lim_{h\to0}\frac{\frac{0}{0+h^2}}{h}=\lim_{h\to0}\frac{0}{h^3}=0[/tex]
Function is also continuous in (0,0) (although it does not affect differentiability). Is there a mistake in this exercise?