*Mathematics*

*Mathematics*

Mathematics is an abstraction. Instead of thinking about particular numbers, mathematics focuses on the underlying patterns.

This note contains basic math formulas and terms that I mix up on a semi-regular basis.

Math:

**Theorem:**A pattern or rule.**Proof:**Calculations that show that a theorem is always true.

Circle:

**Center:**Every point of a circle have the same distance from this point.**Edge:**Points that form the outline of a circle.**Radius:**Distance from the center to the edge.**Circumference:**Distance of the edge.**Diameter:**Distance from an edge point to opposite point. Twice the radius.**Pi:**Ratio between circumference and diameter. Always 3.141559...

**Average (Arithmetic Mean):** The sum of a collection of values divided by the number of values in the collection.

```
sum(x) / x.length
```

**Mid-range:** Average of the minimum and maximum values in a collection.

```
( max(x) + min(x) ) / 2
```

**Median:** The middle value in an ordered collection of value. If there is a even number of numbers, median is average of the two middle numbers.

```
x = [a, b, c, d, e]
^ ^ ^
min. med. max.
```

**Mode:** The most common value in a collection of values.

```
x[a, a, b, c, c, b, b] = Mode is b
```

**Quadratic** is a generalization of conic sections such as ellipses, circles, parabolas and hyperbolas. Think of a 3D cone where you cut slices.

**Transpose** is flipping matrix rows to columns and columns to rows over its the diagonal, `A`

vs `A^T`

.

**Ordinal** data is a categorical data type where the variables have ordered categories and the distances between the categories are not known.