$$f^\prime(x)=\lim_{h \rightarrow 0}{{f(x+h)-f(x)} \over h}$$
+Not differentiable at:
+
+- discontinuous points
+- sharp point/cusp
+- vertical tangents ($\infty$ gradient)
+
## Tangents & gradients
**Tangent line** - defined by $y=mx+c$ where $m={dy \over dx}$