
Suppose that $f\colon \mathbb R\to \mathbb R$ is differentiable at $c$ and that $f(c) := 0 $.
Show that $g(x) := |f(x)|$ is differentiable at $c$ if and only if $f'(c) = 0$.
My attempt:
We know that $f$ is differentiable at $c$ but we do not know what the value of this derivative is so
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