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4. Consider functions

4. Consider functions \(f(x, y)=\sqrt[3]{x^{3}+y^{3}}\) and \(\mathbf{c}(t)=(t, 2 t)\).

(a) Show that \(f_{x}(0,0)\) and \(f_{y}(0,0)\) exist. What is \(\nabla f(0,0)\) ?

(b) Show that \(f\) is not differentiable at the point \((0,0)\).

(c) Find \((f \circ c)(t)\) and then compute its derivative at the point \(t=0\).

(d) Show that \((f \circ \mathbf{c})^{\prime}(0) \neq \nabla f(\mathbf{c}(0)) \cdot \mathbf{c}^{\prime}(0)\). Does this contradict the chain rule formula? why? why not?

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