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August 12th, 2012, 02:17 PM  #1 
Newbie Joined: Aug 2012 Posts: 5 Thanks: 0  The Radius of a Circle
Given this equation: x^2 + y^2  2x  2y = 9 Is this the equation of a circle? It appears to me that this circle has a negative radius, and this does not make sense to me. The square root of 9 is imaginary, and therefore it seems that this equation cannot graph a circle. 
August 12th, 2012, 04:04 PM  #2  
Math Team Joined: Dec 2006 From: Lexington, MA Posts: 3,267 Thanks: 408  Re: The Radius of a Circle Hello, awinston! Quote:
You happen to be correct . . . but for the wrong reason. The minus on the "9" is not the proper indicator. We must complete the square first. [color=beige]. . . . . . . . . . . . . . . . . . . . . . . .[/color] [color=beige]. . . . . . . . . . . . . . [/color] Got it?  
August 12th, 2012, 05:07 PM  #3 
Newbie Joined: Aug 2012 Posts: 5 Thanks: 0  Re: The Radius of a Circle
Yes, of course! Thank you so much. When I apply this to the problem that I'm actually working on, I see that the circle really does exist. Andrew 

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