Linear algebra for machine learning

## 1. Distance between two points

• Let A & B be two points in 2 dimensional geometry.

• #### Euclidean distance

Example: • #### Minkowski distance (Generalisation)

  where p = 1, 2, 3 ...

if p == 1, minkowski distance will converge into Manhatten distance

if p == 2, minkowski distance will converge into Euclidean distance

• If A & B points are in n dimensional geometry, then

Then,

1. Euclidean distance(A,B) = $\sqrt{(x_{a1}-x_{b1})^2 + (x_{a2}-a_{b2})^2+ ... + (x_{an}-x_{bn})^2}$

2. Manhatten distance(A,B) = $|x_{a1}-x_{b1}| + |x_{a2}-a_{b2}| + .... + |x_{an}-a_{bn}|$

3. Minkowski distance(A,B) = $({|x_{a1}-x_{b1}|^p + |x_{a2}-a_{b2}|^p+ ... + |x_{an}-x_{bn}|^p})^{1/p}$

## 2. Dot product and angle between 2 vectors

a & b points are in n dimensional vectors, i.e.,

Dot product of a & b = a.b

It is same as matrix multiplication of a & b vectors, i.e.,

Note: By default, a vector is column vector if not mentioned explicitly. So here a & b are column vectors

Also we know that : $a.b = |a| * |b| * Cos(\theta)$ (proof)

where a = distance of a from origin & b = distance of b from origin

## 3. Equation of a line, plane

Line in 2D = Plane in 3D = hyper plane in nD

• #### 2 dimensional geometry

The equation of a line in 2D is $ax+by+c=0$

which can also be written as $w_1x_1+w_2x_2+c=0$ If line passes through origin (0, 0) then y-intercept becomes 0. So equation becomes

• #### 3 dimensional geometry

Equation of a plane in 3D passing through the origin (0,0)

• #### n dimensional geometry

Equation of a plane in nD passing through the origin (0,0)

i.e.,

We know that if a.b = 0, then a is perpendicular to b ( $\because$ if $Cos(\theta)$ = 0, then $\theta = 90^\circ$)

$\therefore$ $w$ is perpendicular to any point on plane $x$, provided plane passes through origin

## 4. Projection of a vector

Let $a$ & $b$ be two vectors and $\theta$ be the angle between them.

$d$ is the projection of $a$ on $b$ Then

Multiply both sides by $|b|$

We know that $a.b = |a| * |b| * Cos(\theta)$

If $b$ is a unit vector, then $d = a.b$

## 5. Distance of a point from a plane

Let $p$ be any point in $n$ dimensional geometry, which is at a distance $d$ from the hyper plane $\pi_n$.

Let $w$ be a vector passing through origin (0,0) Distance of point $p$ from the plane $\pi$ is the projection of $p$ on $w$

$\therefore$ projection of $p$ on $w$ = $d$

If $w$ is a unit vector, then $d = w.p$

$d$ is positive since $\theta$ is less than 90$^\circ$

Similary we can calculate the distance of the point $p'$ from the plane $\pi_n$. Since $\theta'$ is greater than $90^\circ$, the distance will be negative.

In this way, we can decide in which side of the plane a point lies.

## 6. Equation of a circle

Let $C$ be the center of the circle & $P$ be the locus of the center of the circle. Let $r$ be the radius of the circle

If the center of the cirlce is origin i.e., $(0, 0)$, then the equation of the circle is given as

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