Math Is Fun Forum

waven

Member

Registered: 2009-10-06

Posts: 11

Area of a circle :S

Q.a tablecloth is to cover a circular table that has a diameter of 1.9m. what area of the cloth is needed if the tablecloth is to hang 15cm over the edge of the table ( nearest to cm² ).
thanks

Last edited by waven (2009-12-19 09:33:52)

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Area of a circle :S

The tablecloth is making the shape of a cylinder, with one end missing.

The height is 15cm and the radius is 95cm, so the areas are as follows:

On the table - π * 95² = 28352.87... cm²
Hanging - 15 * 190 * π = 8953.54... cm²

The total area needed is therefore 37306 cm² (to the nearest whole number).

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Why did the vector cross the road?
It wanted to be normal.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Area of a circle :S

Hi;

I have a different interpretation. If the table cloth was currently covering the table with a diameter of 1.9 meters or 190 centimeters. Then if we just add 15 cm more all around then it will hang over the table by 15 cm all around. So:

The diameter increases to 220 cm. A = 110^2 * pi = 38013 cm^2. to the nearest centimeter. Where am I going wrong?

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Area of a circle :S

Our tablecloths are two different shapes. Mine is "purpose-built" to go around the table. It's meant to hang perfectly off a circle, and can't be folded flat very easily.

Yours is made from a piece of flat cloth, and if you hang it off the table then it'll have to fold a bit to fit.

I'm not sure which interpretation is the required one, but that explains why we're getting different answers.

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Why did the vector cross the road?
It wanted to be normal.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Area of a circle :S

Yes, I understand what you are saying.

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.