The second problem set was asking for a specific case of a particular function if it is not differentiable. I could not visualize easily from the text problem alone but the specific problem has a sudden slope change at x=0 but then any function that has no smoothness in at least one instant is already not differentiable in the entire domain but it can still be differentiable at other locations. In theoretical physics we call this singularity of functions.
Currently at the Rate of Change in the Single Variable Calculus.
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