What does a "circle equation" mean?

Question

For instance, (r+1)² + (r-3)²=15 or whatever, I don't get how that works. (Granted, I haven't been taught it either, but now is the time I hope). Thank you.

PS: I think r means radius but I don't know...

Answer 1

 This is the equation of a circle with coordinate of Center at ( -1. 3) and radius √15

 

    To find out draw a circle with radius r.

 

    If you take a point on the perimeter of the circle P, and draw a perpendicular line to the axis call him

     

            P ( X,y), then Pythagoras theorem

 

             X^2 + Y^ 2 = r^2

 

            and then if you use vertical and Horizontal translation Shift the center to point with coordinate of

             ( h, k)

 

                Then equation of circle becomes :

                

               ( X - h )^2 + ( Y - k) ^ 2 = r^2

 

                Which is this case where ( h,k) are ( -1, 3) and radius = √15

 

              Check your text for Conics, and translation of curves.

 

Answer 2

Hi George,

 

It's difficult to teach this over this message board, but after doing a 30-second Google search, this was, in my opinion, the most useful resource for what you're looking to learn.

 

www.mathsisfun.com/algebra/circle-equations.html

 

Good luck!  Do ask us if you have any questions.

 

-Murtaza

 

PS:  in what you wrote, (r+1)² + (r-3)²=15, the r's usually don't go over there... just sayin' ;)