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Sorry for the delay; I've been kind of busy recently.
Yes; I can only think of variations of the fact that a function whose graph contains a corner is not differentiable at the corner, since the tangent space would be generated by the line y=f'(x)x+b.
Yes; I can only think of variations of the fact that a function whose graph contains a corner is not differentiable at the corner, since the tangent space would be generated by the line y=f'(x)x+b.