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If the radii of each of the outer circles is r then show that radius of the inner circle is

*Last edited by Agnishom (2012-11-21 15:03:47)*

'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'

'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'

I'm not crazy, my mother had me tested.

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 7,501

hi Agnishom

See diagram. Let the radius of the little circle be R

AB = BC = r

AC = R + r (why?)

So you should be able to use Pythagoras here.

Bob

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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WOW! Thats a great proof

But what if the examiner asks me How do I know that BC = r (apart from intution)?

'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'

'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'

I'm not crazy, my mother had me tested.

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 7,501

OK. Another diagram below.

This one shows a circle in the middle (B) and one on the left that crosses it (A) and one on the right that just touches it (C) .

The tangent to B where it crosses A cuts A again.

The tangent to B where it touches C is also a tangent for C.

General rule. When two circles touch they share a common tangent.

A tangent is always at right angles to the radius at the point where the tangent touches.

So back to the previous diagram:

ABC = 90. If you give a letter to a fourth point, you'll complete a square. So BC = r

And AD is perpendicular to the common tangent at D (haven't drawn this line) and DC is too.

So ADC is a straight line.

Bob

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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I see

Thanks

However I will like to ask you again if I have any doubts

'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'

'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'

I'm not crazy, my mother had me tested.

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 7,501

Certainly. Geometry and I have a long and happy relationship.

Bob

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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**n872yt3r****Member**- Registered: 2013-01-21
- Posts: 392

Did you steal my idea?

- n872yt3r

Math Is Fun Rocks!

By the power of the exponent, I square and cube you!

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**n872yt3r****Member**- Registered: 2013-01-21
- Posts: 392

Keep this comment up; I'm linking it to the GeoGebra Forum.

There are:

4 Circles

22 Squares

24 Triangles

66 Rectangles

72 Irregular

- n872yt3r

Math Is Fun Rocks!

By the power of the exponent, I square and cube you!

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Hi n872yt3r.

WHAT ARE YOU TALKING ABOUT?

'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'

I'm not crazy, my mother had me tested.

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**n872yt3r****Member**- Registered: 2013-01-21
- Posts: 392

I'm thinking you stole my idea because I did the same thing on a thread of 3 kissing circles with bobbym.

And the 4 circles etc. is for my upload on the GeoGebra Forum and on this site where you can upload GeoGebra programs.

- n872yt3r

Math Is Fun Rocks!

By the power of the exponent, I square and cube you!

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