# Circle Theorems

This article explains circle theorems, including tangents, sectors, angles and proofs (with thanks to Revision Maths).

## Isosceles Triangle

Two Radii and a chord make an isosceles triangle.

## Perpendicular Chord Bisection

The perpendicular from the centre of a circle to a chord will always bisect the chord (split it into two equal lengths).

## Angles Subtended on the Same Arc

Angles formed from two points on the circumference are equal to other angles, in the same arc, formed from those two points.

## Angle in a Semi-Circle

Angles formed by drawing lines from the ends of the diameter of a circle to its circumference form a right angle. So c is a right angle.

### Proof

We can split the triangle in two by drawing a line from the centre of the circle to the point on the circumference our triangle touches.

We know that each of the lines which is a radius of the circle (the green lines) are the same length. Therefore each of the two triangles is isosceles and has a pair of equal angles.

But all of these angles together must add up to 180°, since they are the angles of the original big triangle.

Therefore x + y + x + y = 180, in other words 2(x + y) = 180.
and so x + y = 90. But x + y is the size of the angle we wanted to find.

## Tangents

A tangent to a circle is a straight line which touches the circle at only one point (so it does not cross the circle- it just touches it).

A tangent to a circle forms a right angle with the circle’s radius, at the point of contact of the tangent.

Also, if two tangents are drawn on a circle and they cross, the lengths of the two tangents (from the point where they touch the circle to the point where they cross) will be the same.

## Angle at the Centre

The angle formed at the centre of the circle by lines originating from two points on the circle’s circumference is double the angle formed on the circumference of the circle by lines originating from the same points. i.e. a = 2b.

### Proof

You might have to be able to prove this fact:

OA = OX since both of these are equal to the radius of the circle. The triangle AOX is therefore isosceles and so ∠OXA = a
Similarly, ∠OXB = b

Since the angles in a triangle add up to 180, we know that ∠XOA = 180 – 2a
Similarly, ∠BOX = 180 – 2b
Since the angles around a point add up to 360, we have that ∠AOB = 360 – ∠XOA – ∠BOX
= 360 – (180 – 2a) – (180 – 2b)
= 2a + 2b = 2(a + b) = 2 ∠AXB

## Alternate Segment Theorem

This diagram shows the alternate segment theorem. In short, the red angles are equal to each other and the green angles are equal to each other.

### Proof

You may have to be able to prove the alternate segment theorem:

We use facts about related angles

A tangent makes an angle of 90 degrees with the radius of a circle, so we know that ∠OAC + x = 90.
The angle in a semi-circle is 90, so ∠BCA = 90.
The angles in a triangle add up to 180, so ∠BCA + ∠OAC + y = 180
Therefore 90 + ∠OAC + y = 180 and so ∠OAC + y = 90
But OAC + x = 90, so ∠OAC + x = ∠OAC + y
Hence x = y

cyclic quadrilateral is a four-sided figure in a circle, with each vertex (corner) of the quadrilateral touching the circumference of the circle. The opposite angles of such a quadrilateral add up to 180 degrees.

Area of Sector and Arc Length

If the radius of the circle is r,
Area of sector = πr2 × A/360
Arc length = 2πr × A/360

In other words, area of sector = area of circle × A/360
arc length = circumference of circle × A/360

# Spelling Rules

The problem with the English is that we’ve invaded (and been invaded by) so many countries that our language has ended up with a mish-mash of spelling rules.

English is among the easiest languages to learn but among the most difficult to master. One of the problems is spelling. We have so many loan words from so many different languages that we’ve been left with a huge number of spelling rules – and all of them have exceptions!

Contrast that with Spanish, for example, where what you see is generally what you get. The problem for students of English, then, is that it’s very difficult to find shortcuts to improve your spelling, and an awful lot of words just have to be learned off-by-heart. Considering that there are over a million words in English, that’s a big ask!

There are lots of lists of spelling rules on the web, but I thought I’d put down what I think are the most useful ones.

1. I before E except after C when the sound is /ee/.
This is the most famous rule of English spelling, but there are still exceptions! Hence, we write achieve with -ie- in the middle but also ceiling, with -ei- in the middle, as the /ee/ sound comes after the letter c. The most common exceptions are weird and seize.
2. If you want to know whether to double the consonant, ask yourself if the word is like dinner or diner.
One of the most common problems in spelling is knowing when to double a consonant. A simple rule that helps with a lot of words is to ask yourself whether the word is more like dinner or diner. Diner has a long vowel sound before a consonant and then another vowel (ie vowel-consonant-vowel, or VCV). Words with this long vowel sound only need one consonant before the second vowel, eg  shinerfiver and whiner. However, dinner has a short first vowel and needs two consonants to ‘protect’ it (ie vowel-consonant-consonant-vowel, or VCCV). If the word is like dinner, you need to double the consonant, eg winnerbitter or glimmer. Just bear in mind that this rule doesn’t work with words that start with a prefix (or a group of letters added to the front of a word), so it’s disappoint and not dissapoint.
3. If the word has more than one syllable and has the stress on the first syllable, don’t double any final consonant.
This rule sounds a bit complicated, but it’s still very useful – particularly if it helps you spot your teacher making a mistake! We generally double the final consonant when we add a suffix starting with a vowel, such as -ing, but this rule means we shouldn’t do that as long as a) the word has more than one syllable and b) the stress is on the first syllable, eg focusing and targeted, but progressing and regretting. The main exceptions to this are words ending in -l and -y, hence barrelling and disobeying.
4. When adding a suffix starting with a consonant, you don’t need to change the root word unless it ends in -y. This is among the easiest and most useful rules. There are loads of words ending in suffixes like -less-ment or -ness, but spelling them should be easy as long as you know how to spell the root word, eg shoe becomes shoelesscontain becomes containment and green becomes greenness. However, words ending in -y need the y changing to an i, so happy becomes happiness.
5. When adding a suffix starting with a vowel to a word ending in a silent -e, the must be dropped unless it softens a or a g.
An at the end of a word is often called a ‘Magic E’, as it lengthens the vowel before the final consonant, eg fat becomes fate. However, that job is done by the vowel at the start of the suffix when it is added to the word, so it needs to be dropped, eg race becomes racing and code becomes coded. The main exceptions come when the word ends with a soft or g, which need to be followed by an -e, an -i or a -y to sound like /j/ and /s/ rather than /g/ and /k/. If the suffix doesn’t begin with an e- or an i-, we still need the to make sure the word sounds right, eg managing  is fine without the -e, as the in -ing keeps the soft, but manageable needs to keep the -e to avoid a hard /g/ sound that wouldn’t sound right.
6. The only word ending in -full is full!
There are lots of words ending in what sounds like -full, but the only one that has two ls at the end is full. All the other words – and there are thankfully no exceptions! – end in -ful, eg skilfulbeautiful and wonderful.