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In school, we learn how a line has an equation… and a circle has an equation… what does this mean?

*The short answer is*

points on curve solutions of equation

*however this note explains all of this from first principles, with a particular emphasis on the set-theoretic fundamentals.*

## Set Theory

A *set** *is a collection of objects. The objects of a set are referred to as the *elements* or *members** *and if we can list the elements we include them in curly-brackets. For example, call by the set of whole numbers (strictly) between two and nine. This set is denoted by

.

We indicate that an object is an element of a set by writing , said, *in * or *is an element of *. We use the symbol to indicate non-membership. For example, .

Elements are not duplicated and the order doesn’t matter. For example:

.

This post follows on from this post where the following principle was presented:

Fundamental Principle of Solving ‘Easy’ Equations

Identify what is difficult or troublesome about the equation and get rid of it. As long as you do the same thing to both numbers (the “Lhs” and the “Rhs”), the equation will be replaced by a simpler equation with thesamesolution.

There are a number of subtleties here: basically sometimes you get extra ‘solutions’ (that are not solutions at all), and sometimes you can lose solutions.

Let us write the squaring function, e.g. , by and the square-rooting function by . It appears that are an inverse pair but not quite exactly. While

and

,

check out O.K. note that

,

does not bring us back to where we started.

This problem can be fixed by restricting the allowable inputs to to positive numbers only but for the moment it is better to just treat this as a subtlety, namely while , … in fact I recommend that we remember that with an there will generally be *two *solutions.

The other thing we look out for as much as possible is that *we cannot divide by zero*.

There are other issues around such as the fact that , so that the equation has no solutions (no, is not a solution! Check.). This equation has *no *solutions.

Often, in context, these subtleties are not problematic. For example, equations with no solutions rarely arise and quantities might be positive so that if we have , only need be considered (for example, might be a length).

In this short note we will explain why we multiply matrices in this “rows-by-columns” fashion. This note will only look at matrices but it should be clear, particularly by looking at this note, how this generalises to matrices of arbitrary size.

First of all we need some objects. Consider the plane . By fixing an origin, orientation (– and -directions), and scale, each point can be associated with an ordered pair , where is the distance along the axis and is the distance along the axis. For the purposes of linear algebra we denote this point by

.

We have two basic operations with points in the plane. We can add them together and we can scalar multiply them according to, if and :

, and

.

Objects in mathematics that can be added together and scalar-multiplied are said to be *vectors. *Sets of vectors are known as *vector spaces* and a feature of vector spaces is that all vectors can be written in a unique way as a sum of *basic vectors. *

A nice little question:

*Given a regular pentagon with side length *, *what is the relationship between the area and the side-length?*

First of all a pentagon:

We use triangulation to cut it into a number of triangles:

With in each of the three triangles, there in those angles around the edges, and, as there are five of them, they are each .

Next triangulate from the centre. With a plain oul pentagon we might not be sure that such a centre exists but if you start with a circle and inscribe five equidistant points along the circle, the centre of the circle serves as this centre:

As everything is symmetric, each of these triangles are the same and the ‘rays’ are also the same as they are all radii. The angle at the centre is equal to , and furthermore, by symmetry, the rays bisect the larger angles and so each of these triangles are .

Using radians, because they are nicer, . Note that, where is the perpendicular height:

.

A problem for another day is finding the exact value of . It is

Therefore the area of one such triangle is:

,

Therefore the area of the pentagon is five times this:

,

with . It might be possible to simply further.

Quadratics are ubiquitous in mathematics. For the purposes of this piece a quadratic is a real-valued function of the form

,

where such that . There is a little bit more to be said — particularly about the differences between a quadratic and a quadratic *function *but for those this piece is addressed to (third level: non-maths; all second level), the distinction is unimportant.

## Geometry

The basic object we study is the square function, , :

All quadratics look similar to . If then the quadratic has this geometry. Otherwise it looks like and has geometry

The geometry dictates that quadratics can have either zero, one or two real roots. A root of a function is an input such that . As the graph of a function is of the form , roots are such that , that is where the graph cuts the -axis. With the geometry of quadratics they can cut the -axis no times, once (like ), or twice.

There are a number of ways of explaining why you cannot divide by zero. Here are my two favourites.

## Any Set of Numbers Collapses to a Single Number

How old are you? Zero years old.

How tall are you? Zero metres old.

How many teeth do you have? Zero.

How many Superbowls has Tom Brady won? Zero

Yep, if you allow division by zero you only end up with one number to measure everything with.

After a long time I have finally completed my PhD studies when I handed in my hardbound thesis (a copy of which you can see here).

It was a very long road but thankfully now the pressure is lifted and I can enjoy my study of quantum groups and random walks thereon for many years to come.

As I said in the previous post, there is a duality:

Points on a Curve (Geometry) Solutions of an Equation (Algebra)

This means we can answer geometric questions using algebra and answer algebraic questions using geometry.

### Problem

Consider the following two questions:

Find the tangents to a circle of a given slope.Find the tangents to a circle through a given point.

Both can be answered using the duality principle.

#### Example

*Find the tangents to the circle*

*that are*

(a)

parallel to the line(b)

through the point[caution: the numbers here are disgusting]

*Solution (a) i:*

* *First of all a sketch (and the remark that a tangent is a line):

*Here we see the circle and the line on the bottom left. The two tangents we are looking for are as shown. They have the same slope as and have only one intersection with . These two pieces of information will allow us to find the equations of the tangents.*

Arguably, the three central concepts in the theory of differential calculus are that of a function, that of a tangent and that of a limit. Here we introduce functions and tangents.

## Functions

When looking at differential calculus, two good ways to think about functions are via algebraic geometry and interdependent variables. Neither give the proper, abstract, definition of a function, but both give a nice way of thinking about them.

### Algebraic Geometry Approach

Let us set up the plane, . We choose a distinguished point called the origin and a distinguished direction which we call ‘positive ‘. Draw a line through the origin in the direction of positive . This is the -axis. Choose a unit distance for the -direction.

Now, perpendicular to the -axis, draw a line through the origin. This is the -axis. By convention positive is anti-clockwise of positive . Choose a unit distance for the -direction.

This is the plane, :

Now points on the plane can be associated with a pair of numbers . For example, the point a distance one along the positive and five along the negative can be denoted by the coordinates (1,-5):

Similarly, I can take a pair of numbers, say (-1,3), and this corresponds to a point on the plane.

This gives a duality:

points on the plane pairs of numbers

Now consider the completely algebraic objects

.

This post follows on from this post where the logic for the below is discussed. I am not going to define here what easy means!

Here is the strategy/guiding principle:

Fundamental Principle of Solving ‘Easy’ Equations

Identify what is difficult or troublesome about the equation and get rid of it. As long as you do the same thing to both numbers (the “Lhs” and the “Rhs”), the equation will be replaced by a simpler equation with thesamesolution.

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