It was really tough for me to understand many articles about data science due to the requirements of understanding mathematics (especially linear algebra). I’ve started to gain some basic knowledges about Math by reading a book first.

The great tool Typora and stackedit with supporting Katex syntax simply helps me to display Math-related symbols.

Let’s start!

*The fundamental ideas of mathematics: “doing math” with numbers and functions. Linear algebra: “doing math” with vectors and linear transformations.*

### 1. Solving equations

Solving equations means finding the value of the *unknown* in the equation. To find the solution, we must break the problem down into simpler steps. E.g:

$\begin{aligned} x^2 - 4 &= 45\\ x^2 - 4 + 4 &= 45 + 4\\ x^2 &= 49\\ \sqrt{x}&=\sqrt{49}\\ |x| &= 7\\ x=7 &\text{ or } x=-7 \end{aligned}$

### 2. Numbers

**Definitions**

Mathematicians like to classify the different kinds of number-like objects into *sets*:

- The natural numbers: $N$ = {$0, 1, 2, 3, 4, 5, 6, 7$, … }
- The integer: $Z$ = { … , $-3, -2, -1, 0, 1, 2, 3$, … }
- The rational numbers: $Q$ = {${5}\over{3}$, ${22}\over{7}$, $1.5, 0.125, -7$, … }
- The real numbers: $R$ = {$-1, 0, 1, \sqrt{2}, e, \pi, 4.94...$, … }
- The complex numbers: $C$ = {$-1, 0, 1, i, 1 + i, 2 + 3i$, … }

**Operations on numbers**

**Addition**is commutative and associative. That means:

$a + b = b+ a$

$a + b + c = (a + b) + c = a + (b + c)$**Subtraction**is the inverse operation of addition.**Multiplication**is also commutative and associative.

$ab = \underbrace{a + a + a + ... + a}_{\text{b times}} = \underbrace{b + b + b + ... + b}_{\text{a times}}$

$ab = ba$

$abc = (ab)c = a(bc)$**Division**is the inverse operation of multiplication. You cannot divide by 0.**Exponentiation**is multiplying a number by itself many times.

$a^b = \underbrace{aaa...a}_{\text{b times}}$

$a^{-b} = {{1}\over{a^b}}$

$\sqrt[n]{a} \equiv a^{{1}\over{n}}$

The symbol “$\equiv$” stands for “is equivalent to” and is used when two mathematical object are identical.

### 3. Variables

*Variables* are placeholder names for any number or unknown. *Variable substitution*: we can often change variables and replace one unknown variable with another to simplify an equation. For example:

$\begin{aligned} {6 \over{5 - \sqrt{x}}} = \sqrt{x}\\ u = \sqrt{x}\\ {6 \over{5 - u}} = u \end{aligned}$

### 4. Functions and their inverses

The *inverse function* $f^{-1}$ performs the opposite action of the function $f$ so together the two functions cancel each other out. For example:

- $f(x) = c$
- $f^{-1}(f(x)) = x = f^{-1}(c)$
- $x=f^{-1}(c)$

Common functions and their inverses:

$\begin{aligned}
function f(x) &\Leftrightarrow inverse f^{-1}(x)\\
x+2 &\Leftrightarrow x-2\\
2x &\Leftrightarrow {1\over2}x\\
x^2 &\Leftrightarrow \pm{\sqrt{x}}\\
2^x &\Leftrightarrow log{_2}(x)\\
3x + 5 &\Leftrightarrow {1\over 3}(x-5)\\
a^x &\Leftrightarrow log{_a}(x)\\
exp(x) \equiv e^x &\Leftrightarrow ln(x) \equiv log{_e}(x)\\
sin(x) &\Leftrightarrow sin^{-1}(x) \equiv arcsin(x)\\
cos(x) &\Leftrightarrow cos^{-1}(x) \equiv arccos(x)
\end{aligned}$

The principle of “digging” (Bruce Lee-style) toward the unknown by applying inverse functions is the key for solving all these types of equations, so be sure to practice using it.

### 5. Basic rules of algebra

Given any three numbers a, b, and c we can apply the following algebraic properties:

- Associative property: $a + b + c = (a + b) + c = a + (b+ c)$ and $abc = (ab)c = a(bc)$
- Commutative property: $a + b = b + a$ and $ab = ba$
- Distributive property: $a(b + c)$ = $ab + ac$

Some algebraic tricks are useful when solving equations

- Expanding brackets: $(x + 3)(x +2) = x^2 + 5x + 6$
- Factoring: $2x^2y + 2x + 4x = 2x(xy + 1 + 2) = 2x(xy + 3)$
- Quadratic factoring: $x^2-5x+6=(x-2)(x-3)$
- Completing the square: $Ax^2 + Bx + C = A(x- h)^2 + k$ e.g: $x^2 + 4x + 1 = (x + 2)^2-3$

### 6. Solving quadratic equations

The solutions to the equation $ax^2 + bx + c =0$ are

$x_1 = {{-b + \sqrt{b^2 - 4ac}}\over{2a}} \quad and \quad x_2 = {{-b -\sqrt{b^2-4ac}}\over{2a}}$

Actually, we can use the technique `completing the square`

to explain this formula.

### 7. The Cartesian plane

**Vectors and points**

- Point: $P = (P_x, P_y)$. To find this point, start from the origin and move a distance $P_x$ on the x-axis, then move a distance $P_y$ on the y-axis.
- Vector: $\overrightarrow{v} = (v_x, v_y)$. Unlike points, we don’t necessarily start from the plane’s origin when mapping vectors.

**Graphs of functions**

The Cartesian plane is great for visualizing functions, $f: {R} \rightarrow {R}$

A function as a set of input-output pairs $(x, y) = (x, f(x))$

### 8. Functions

We use functions to describe the relationship between variables.

To “know” a function, you must be able to understand and connect several of its aspects including definition, graph, values and relations.

Definition: $f: A \rightarrow B$. Function is a mapping from numbers to numbers.

- Function composition: $fog(x)\equiv f(g(x)) = z$
- Inverse function: $f^{-1}(f(x)) \equiv f^{-1}o f(x) = x$
- Table of values: $\{(x1, f(x1)), (x2, f(x2)), ...\}$
- Function graph: using the Cartesian plane
- Relations: e.g: $sin^2x + cos^2x = 1$

### 9. Function references

**- Line**

The equation of a line: $f(x) = mx + b$ and $f^{-1} (x) = {{1\over m} (x-b)}$

The general equation: $Ax + By = C$

**- Square/Quadratic**: $f(x) = x^2$

**- Square root**: $f(x) = \sqrt x \equiv x ^{1\over2}$

**- Absolute value**: $f(x) = |x| = \begin{cases} x &\text{if } x \ge 0, \\ c &\text{if } x \lt 0.\end{cases}$

**- Polynomial functions**: $f(x) = a_0 + a_1x + a_2x^2 + a_3x^3 + ... + a_nx^n$

**- Sine**: $f(x) = sin(x)$

**- Consine**: $f(x) = cos(x) = sin(x + {\pi\over2})$

**- Tangent**: $f(x) = tan(x) \equiv {sin(x)\over{cos(x)}}$

**- Exponential**: $f(x) = e^x \equiv exp(x)$

**- Natural logarithm**: $f(x) = ln(x) = log_e(x)$

**- Function transformation**

Vertical translation: $g(x) = f(x) + k$

Horizontal translation: $g(x) = f(x-h)$

Veritcal scaling: $g(x) = Af(x)$

Horizontal scaling: $g(x) = f(ax)$

**- General quadratic function**: $f(x) = A(x-h)^2 + k$

**- General sine function**: $f(x) = Asin({2\pi\over\lambda}x - \phi)$

### 10. Polynomials

In general, a polynomial of degree $n$ has the equation

$f(x) = a_nx^n + a_{n-1}x^{n-1} + ...+ a_2x^2 + a_1x + a_0 \equiv \displaystyle\sum_{k=0}^na_kx^k$

### 11. Trigonometry

Pythagoras’ theorem

$\begin{aligned}
|adj|^2 + |opp|^2 &= |hyp|^2\\
{{|adj|^2}\over{|hyp|^2}} + {{|opp|^2}\over{|hyp|^2}} &= 1\\
sin^2(\theta) + cos^2(\theta) &= 1
\end{aligned}$

### 12. Trigonometric identities

$sin^2(\theta) + cos^2(\theta) = 1$

$sin(a+ b) = sin(a)cos(b) + sin(b)cos(a)$

$cos(a+b) = cos(a)cos(b) - sin(a)sin(b)$

And, more …

### 13. Geometry

A: area, P: perimeter, V: volume

Triangles: $A = {1\over2}ah_a$, $P = a + b + c$

Sphere: $A=4\pi r^2$, $V={{4\over3}\pi r^3}$

Cylinder: $A = 2(\pi r^2) + (2\pi r)h$, $V = (\pi r^2)h$

### 14. Circle

Radians: $2\pi [rad] = 360^0$

### 15. Sovling systems of linear equations

$\begin{aligned}
a_1x + b_1y = c1\\
a_2x + b_2y=c_2
\end{aligned}$

There are some approaches to sovling it:

- Solving by substitution
- Solving by substraction
- Solving by equating

Reference:

[1]. **Ivan Savov**, “No bullshit guide to linear algebra”.

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