math
Sine Law
$$\frac{\sin(\text{A})}{\text{a}} = \frac{\sin(\text{B})}{\text{b}} = \frac{\sin(\text{C})}{\text{c}}$$
Finite Difference Approximations
$$\frac{f(x+h) - 2f(x) + f(x-h)}{h^2}$$
Curl
$$\scriptsize \nabla \times \mathbf{F} = \begin{vmatrix} \mathbf{i} & \mathbf{j} & \mathbf{k} \\\\ \frac{\partial}{\partial x} & \frac{\partial}{\partial y} & \frac{\partial}{\partial z} \\\\ F_x & F_y & F_z \end{vmatrix}$$
Divergence
$$\nabla \cdot \mathbf{F} = \frac{\partial F_x}{\partial x} + \frac{\partial F_y}{\partial y} + \frac{\partial F_z}{\partial z}$$
Gradient
$$\nabla f = \frac{\partial f}{\partial x} \mathbf{i} + \frac{\partial f}{\partial y} \mathbf{j} + \frac{\partial f}{\partial z} \mathbf{k}$$
Matrix Multiplication
$$\scriptsize \begin{pmatrix} a & b \\ c & d \end{pmatrix} \begin{pmatrix} w & x \\ y & z \end{pmatrix} = \begin{pmatrix} aw + by & ax + bz \\ cw + dy & cx + dz \end{pmatrix}$$
Quadric Surfaces
$$\\ {x^2 \over a^2} + {y^2 \over b^2} + {z^2 \over c^2} = 1$$
Equation of a Plane
$$ax + by + cz = d$$
Equation of a Sphere
$$\small R^2 = (x-x_0)^2 + (y-y_0)^2 + (z - z_0)^2$$
Fourier + Inverse Fourier Transform
$$X(\omega) = \int_{-\infty}^{\infty}x(t)e^{-j\omega t}\,dt$$
Arithmetic Gradients
$$\scriptsize P = A \left[\frac{(1+i)^n-1}{i(1+i)^n}\right] + G \left[\frac{(1+i)^n-in-1}{i^2(1+i)^n}\right]$$
Equivalent Uniform Annual Cost
$$A = P \left[\frac{i(1 + i)^n}{(1 + i)^n - 1}\right]$$
Present and Future Value
$$F = P(1 + i)^n$$
Feynmann's Trick For Exponential Integrals
$$\int_0^{\infty} x^n e^{-tx} dx = \frac{n!}{t^{n+1}}$$
Finite and Infinite Geometric Sums
$$S_n = \frac{a(1-r^n)}{1-r}$$
Convolution
$$(f*g)(t)=\int_{-\infty}^{\infty} f(\tau)g(t-\tau)d\tau$$
L'Hopital's Rule
$$\lim_{x \to c} \frac{f(x)}{g(x)} = \lim_{x \to c} \frac{f'(x)}{g'(x)}$$
Determinants of Matrices
$$\det(A)$$
Double Angle Trig Identities
$$\sin(2\theta) = 2 \cdot \sin(\theta) \cdot \cos(\theta)$$
Trig Angle Sum/Difference Identities
$$\scriptsize \sin(a \pm b) = \sin(a) \cos(b) \pm \cos(a) \sin(b)$$
Vector Projection
$$proj_{\vec{b}}(\vec{a})=\frac{\vec{a}\cdot\vec{b}}{\|\vec{b}\|^2}\vec{b}=(\vec{a}\cdot\hat{b})\hat{b}$$
Hyperbolic Functions
$$\sinh(x) = \frac{e^x - e^{-x}}{2}$$
Cross Product
$$\vec{a} \times \vec{b} = \begin{vmatrix} \vec{i} & \vec{j} & \vec{k} \\ a_1 & a_2 & a_3 \\ b_1 & b_2 & b_3 \end{vmatrix}$$
Dot Product
$$\vec{a} \cdot \vec{b} = \sum_{i=1}^n a_i b_i$$
Cosine Law
$$c^2 = a^2 + b^2 - 2 \cdot a \cdot b \cdot \cos(\theta)$$
Definition of the Derivative
$$\frac{d}{dx}f(x) = \lim_{h\to 0}\frac{f(x+h)-f(x)}{h}$$
Pythagorean Trig Identities
$$\sin^2(x) + \cos^2(x) = 1$$
Cauchy's Integral Theorem
$$\oint_C f(z)dz = 0$$
Laplace's Equation
$$\nabla^2 f = \nabla \cdot \nabla f = 0$$
Fundamental Theorem of Calculus
$$\int_a^b f(x) dx = F(b) - F(a)$$