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Circle How do you prove that two circles...

SomethingTypical

Posted 07 December 2004 - 08:01 PM

How do you prove that two circles just touch each other?

Thanks

Dave

Posted 07 December 2004 - 08:02 PM

well if they touch each other then the 2 equation equal eac other at that point

SomethingTypical

Posted 07 December 2004 - 08:04 PM

QUOTE(Dave @ Dec 7 2004, 09:02 PM)
well if they touch each other then the 2 equation equal eac other at that point

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Makes sense, thanks.

Justboy

Posted 08 December 2004 - 04:15 PM

Ah, I remember that! I actually keep up to date on the higher by helping a friend who does higher at the moment.

Clare16

Posted 08 December 2004 - 06:50 PM

hey. its to do with the radius of the 2 circles maybe? find the centres nd then do sum stuff! lol- sorry im so vague- reely in a rush!!

Infinite_Dreams

Posted 08 December 2004 - 09:51 PM

I did it by:

If the sum of the two radii = the distance from one center to another, then they touch at one point.

If the sum of the two radii > the distance from one center to another, then they touch at two points.

If the sum of the two radii < the distance from one center to another, then they don't touch.

Discogirl17

Posted 08 December 2004 - 10:10 PM

That is infact how you do it, remember if the distance isnt just a horizontal or vertical line you can form a sort of right-angeld triangle between the two centres with distance (d) as the hypotenuse and use pythagoras to calculate it.

Infinite_Dreams

Posted 09 December 2004 - 05:41 PM

I'd use the distance formula. Eg. if one circle (arbitrarily) had equation (x-4)^2 + (y+1)^2 = 25 and the other (also arbitrarily) (x+1)^2 + (y-1)^2 = 16 then I'd find the center of both circles using (-g, -f) -> (-1, 1) and (4, -1). Use distance formula, the compare this with the sum of the radii (will be 9 here).

fresh graduate

Posted 10 December 2004 - 02:15 PM

[B][I][FONT=Times][SIZE=7][COLOR=purple]Actually I think it’s the right answer to this question wink.gif

fresh graduate

Posted 10 December 2004 - 02:18 PM

smile.gif i mean Infinite Dreams answer

Infinite_Dreams

Posted 10 December 2004 - 08:36 PM

Haha...what's the chances!

Amo

Posted 27 January 2006 - 10:36 AM

Hi, just a quick question... when you are asked to write down the equation of a circle in the exam, can you leave it in the form (x-a)^2+(y-b)^2=r^2, or do you have to expland the brackets?
Thanks

Ally

Posted 27 January 2006 - 11:37 AM

QUOTE(Amo @ Jan 27 2006, 10:36 AM)
Hi, just a quick question... when you are asked to write down the equation of a circle in the exam, can you leave it in the form (x-a)^2+(y-b)^2=r^2, or do you have to expland the brackets?
Thanks

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I think it's fine as it is.

Joel

Posted 28 January 2006 - 03:59 PM

QUOTE(Amo @ Jan 27 2006, 10:36 AM)
Hi, just a quick question... when you are asked to write down the equation of a circle in the exam, can you leave it in the form (x-a)^2+(y-b)^2=r^2, or do you have to expland the brackets?
Thanks

View Post


You only need to expand the brackets if the question asks you to give the equation of the circle in expanded form.


Amo

Posted 28 January 2006 - 06:53 PM

Ok thanks! smile.gif

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