$L^+$ - limit from above
-$\lim_{x \to a} f(x)$ - limit of a point
+$\lim_{x \to a} f(x)$ - limit of a point
- Limit exists if $L^-=L^+$
- If limit exists, point does not.
$$\lim_{x \rightarrow \infty}{{2x+3} \over {x-2}}={{2+{3 \over x}} \over {1-{2 \over x}}}={2 \over 1} = 2$$
-
## Continuous functions
A function is continuous if $L^-=L^+=f(x)$ for all values of $x$.
+
+## Gradients of secants and tangents
+
+Secant (chord) - line joining two points on curve
+
+Tangent - line that intersects curve at one point
+
+given $P(x,y) \quad Q(x+\delta x, y + \delta y)$:
+gradient of chord joining $P$ and $Q$ is ${m_{PQ}}={\operatorname{rise} \over \operatorname{run}} = {\delta y \over \delta x}$
+
+As $Q \rightarrow P, \delta x \rightarrow 0$. Chord becomes tangent (two infinitesimal points are equal).
+
+Can also be used with functions, where $h=\delta x$.
+
+## First principles derivative
+
+$$f^\prime(x) = \lim_{\delta x \rightarrow 0}{\delta y \over \delta x}={dy \over dx}$$
+
+$$m_{\operatorname{tangent}}=\lim_{h \rightarrow 0}f^\prime(x)$$
+
+
+
+$$m_{\operatorname{chord PQ}}=f^\prime(x)$$
+
+first principles derivative:
+$${m_{\operatorname{tangent at P}} =\lim_{h \rightarrow 0}}{{f(x+h)-f(x)}\over h}$$
+
+## Gradient at a point
+
+Given point $P(a, b)$ and function $f(x)$, the gradient is $f^\prime(a)$
+
+
+## Derivatives of $x^n$
+
+For $f: \mathbb{R} \rightarrow \mathbb{R}$ where $f(x)=x^n, x \in \mathbb{N}$
+
+Derivative is $f^\prime(x) = nx^{n-1}$
+
+If $x=$ constant, derivative is $0$
+
+If $f(x)={1 \over x}=x^{-1}, \quad f^\prime(x)=-1x^{-2}={-1 \over x^2}$
+
+If $f(x)=^5\sqrt{x}=x^{1 \over 5}, \quad f^\prime(x)={1 \over 5}x^{-4/5}={1 \over 5 \times ^5\sqrt{x^4}}$
+
+If $f(x)=(x-b)^2, \quad f^\prime(x)=2(x-b)$
+
+$$f^\prime(x)=\lim_{h \rightarrow 0}{{f(x+h)-f(x)} \over h}$$
+$$=\lim_{h \rightarrow 0}
+
+## Euler's number as a limit
+
+$$\lim_{h \rightarrow 0} {{e^h-1} \over h}=1$$