My Math Forum Negative Radius Circle

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 February 17th, 2013, 02:55 PM #1 Newbie   Joined: Feb 2013 From: Montréal, Canada Posts: 6 Thanks: 0 Negative Radius Circle Good day, Just joined this forum and am curious if there are some here who might like to help extend the thinking of one of my young gifted students. We got into a discussion about the radius of circles and he asked what a circle with a radius of -1 would look like? Of course, I resisted the impulse to squash his curiosity and have instead asked him to explore the topic until we meet again. I find young mathematicians gain a deeper understanding when they explore these topics for themselves, so I'm not asking for an "answer" but some trails he might follow to satisfy his own curiosity. Perhaps into topology(?) If you had suggestions for theories / references / applets etc. that might help him explore this topic, I would love to collate them and present them back to him (and you). Thanks in advance, Tom
 February 17th, 2013, 06:18 PM #2 Senior Member   Joined: Dec 2012 Posts: 450 Thanks: 0 Re: Negative Radius Circle There is no circle with a negative radius,because negative numbers do not exist in real life. You cannot say I have -1 pen,but you can say I got -1 pen meaning that you lost one. So "a circle having radius -1" is not a good statement.You can say the circle's radius increased by -1 meaning it was reduced by 1.You cannot possess a negative number,neither can anything.You can either get it or lose it.
February 18th, 2013, 04:10 AM   #3
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 Originally Posted by tfullerton We got into a discussion about the radius of circles and he asked what a circle with a radius of -1 would look like?
The circle of radius 1 centred at $(a,b)$ is $(x-a)^2+(y-b)^2=1^2=1$. The circle of radius ?1 centred at the same point is $(x-a)^2+(y-b)^2=(-1)^2=1$. So the two circles are the same.

 February 18th, 2013, 04:17 AM #4 Math Team   Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 14,597 Thanks: 1038 Re: Negative Radius Circle ...if you kick the ball in your own net during a soccer game, did you score -1 goals ? :P
February 18th, 2013, 04:51 AM   #5
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 Originally Posted by mathmaniac There is no circle with a negative radius,because negative numbers do not exist in real life. You cannot say I have -1 pen,but you can say I got -1 pen meaning that you lost one. So "a circle having radius -1" is not a good statement.You can say the circle's radius increased by -1 meaning it was reduced by 1.You cannot possess a negative number,neither can anything.You can either get it or lose it.
Thank you for your response. I'll ask my student where in life he might find negative numbers. I suspect he will respond with references to temperature, elevation and so on.
Similar discussion around the concept of zero. Is it a number? A value? Or can we only ever achieve a closeness? But that would best start a new thread I think.....

What I'm trying to do is to inspire in this young gifted student a curiosity to explore mathematical concepts, not to tell him "the" answer (in part because I don't think there is one in this case, but more because I want to encourage in him an inquiry disposition).

February 18th, 2013, 04:52 AM   #6
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 Originally Posted by Denis ...if you kick the ball in your own net during a soccer game, did you score -1 goals ? :P
Ha! Thanks, I'll pass that one along

 February 18th, 2013, 04:53 AM #7 Member   Joined: Jan 2013 Posts: 93 Thanks: 0 Re: Negative radius circle Luckily for sports fans soccer wasn’t invented by mathematicians.
February 18th, 2013, 05:46 AM   #8
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Originally Posted by Crimson Sunbird
Quote:
 Originally Posted by tfullerton We got into a discussion about the radius of circles and he asked what a circle with a radius of -1 would look like?
The circle of radius 1 centred at $(a,b)$ is $(x-a)^2+(y-b)^2=1^2=1$. The circle of radius ?1 centred at the same point is $(x-a)^2+(y-b)^2=(-1)^2=1$. So the two circles are the same.
I think I'll ask him what happens when he graphs this using Geonext. I'm hoping he can begin to visualize algorithms and move from theoretical to applied (and back again!).
Appreciate your help inspiring this young mathematician,
Tom

 February 18th, 2013, 06:00 AM #9 Newbie   Joined: Feb 2013 Posts: 5 Thanks: 0 Re: Negative Radius Circle Hy, Well, the radius of the circle is the distance from the center of the circle to its points. A metric (or distance) "d" in metric spaces is defined as a function from the used set A, and the set of real numbers R, that satisfies the following conditions : 1. d(x, y) = 0 if and only if x = y (identity of indiscernibles, or coincidence axiom) 2. d(x, y) = d(y, x) (symmetry) 3. d(x, z) ? d(x, y) + d(y, z) (subadditivity / triangle inequality). now I will demonstrate that 0 ? d(x,y) for any x and y in the set A: assume that there exists x and y such as : d(x,y) < 0 cond2 gives us d(x,y) + d(y,x) < 0 cond3 gives us d(x,x) ? d(x,y) + d(y,x) < 0 cond1 gives us 0 = d(x,x) ? d(x,y) + d(y,x) < 0 then 0 < 0 which is contradiction, then with the said conditions, we can say that d(x, y) >= 0 for any x and y in the set A. Your student should either think of a way to redefine the circle, or to redefine the distance, to make a generalization. For me, the most simple generalization would be redefining the circle, and taking into account the 3rd dimension, by saying that a circle has an orientation, it is either looking "up" or "down" following whether the radius is positif or negatif, in an oriented plan. And don't tell me that "up" and "down" are not defined and corresponds to from where we look !!! I said oriented plan. cheers
 February 18th, 2013, 06:18 AM #10 Newbie   Joined: Feb 2013 Posts: 5 Thanks: 0 Re: Negative Radius Circle Or maybe a circle with positif radius would be looking inside, and with negatif radius would be looking outside, so the area of a circle with R > 0 would be Pi*R², but if R < 0 it would be +infinity

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# can circle have a negative radius.

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