Math Is Fun Forum

{7/3}

Member

Registered: 2013-02-11

Posts: 210

Another geometry proof

This proof isn't actually needed by me,but i am curious how to prove it,"the diameter of a circle divides the circle into 2 equal parts"

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Agnishom

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From: Riemann Sphere

Registered: 2011-01-29

Posts: 24,996

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Re: Another geometry proof

Define a diameter

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Bob

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Registered: 2010-06-20

Posts: 10,619

Re: Another geometry proof

hi {7/3}

Back to Euclid to see what he made of this.  In book 1 of The Elements he has it as an axiom.

17. And a diameter of the circle is any straight-line, being drawn through the center, and terminated in each
direction by the circumference of the circle. (And) any such (straight-line) also cuts the circle in half.

The second part doesn't actually follow directly from the first so I guess a proof is still needed.  I'll give it some thought. 

Bob

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{7/3}

Member

Registered: 2013-02-11

Posts: 210

Re: Another geometry proof

Never mind,i've found a proof.

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Agnishom

Real Member

From: Riemann Sphere

Registered: 2011-01-29

Posts: 24,996

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Re: Another geometry proof

Please post the proof

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'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.

{7/3}

Member

Registered: 2013-02-11

Posts: 210

Re: Another geometry proof

(sorry,can't post pics,somethings wrong with my browser and my english is also shaky)
if n diameters are drawn in a circle than 2n triangles are drawn,n on each side of a specific diameter.for every triangle there is a congruent triangle on other side(2 sides are radii,interior angles are vertical) thus n triangles on one side are congruent to n on the other side.as n tends to infinity the area becomes equal to other sides area.so, both sides have equal area.

Last edited by {7/3} (2013-06-04 03:36:00)

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There are 10 kinds of people in the world,people who understand binary and people who don't.

Agnishom

Real Member

From: Riemann Sphere

Registered: 2011-01-29

Posts: 24,996

Website

Re: Another geometry proof

That is still not clear. Could you give a link?

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'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.

{7/3}

Member

Registered: 2013-02-11

Posts: 210

Re: Another geometry proof

Sorry,i didn't find it in the net,i kinda discovered it.

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anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: Another geometry proof

Hm, can you prove that it will  tend to the area as n goes to infinity.

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{7/3}

Member

Registered: 2013-02-11

Posts: 210

Re: Another geometry proof

I discovered another proof,the angle standing on a diameter is right,so the angles standing on both side of diameter are right and equal.and equal angles in a circle are stand on similar amd equal arc (congruent arcs).so,the arcs on both side of diameters are equal and it follows the sides are equal in area.

Last edited by {7/3} (2013-06-08 23:18:32)

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There are 10 kinds of people in the world,people who understand binary and people who don't.

{7/3}

Member

Registered: 2013-02-11

Posts: 210

Re: Another geometry proof

Hm, can you prove that it will  tend to the area as n goes to infinity.

my textbook uses the fact that as n tends to infinity sum of the triangles' base equals to circumference.I'd love to prove it but i'm busy cause my exams have started...

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There are 10 kinds of people in the world,people who understand binary and people who don't.

{7/3}

Member

Registered: 2013-02-11

Posts: 210

Re: Another geometry proof

Okay,my exams have ended,as n tends to infinity the bases will become infinitesimal,it can be proved using law of cosines.and as the n tends to infinity angle will become small and arc will become small,so both arc and base will become equal and it follows that area will be equal

Last edited by {7/3} (2013-06-24 00:22:13)

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