Let f(x) be a differentiable function on R. (a) If f(a) ̸= 0 for some fixed number a, then |f(x)| is?
differentiable at x = a.
(b) Is it true that if f(a) = 0, then |f(x)| is not differentiable at a? If it is, prove it. If not, give a counter example.
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use Riemann's prove