Definition:
A function $f$ is said to be differentiable at $x=a$
if and only if $$f'(a)=\lim_{h \to 0}
\frac{f(a+h)-f(a)}{h}$$ exists. A function $f$ is said
to be differentiable on an interval $I$ if $f'(a)$
exists for every point $a \in I$.
Differentiability and Continuity
Theorem:
If a function is differentiable at $a$, then it is also
continuous at $a$.
However, it can be continuous without being differentiable!
How can a function fail to be differentiable?
There are several ways that a function can fail to be
differentiable. In particular:
The function may have a discontinuity, e.g.,
the function below at $x=-1$
The function may have a sharp change in direction,
e.g., $f(x) = |x|$ at $x=0$.
The function may have a vertical tangent,
e.g., $f(x) = x^{1/3}$ at $x=0$.