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# Can someone explain to me the equation of a circle? watch

1. I've heard two different formulae,

x^2 + y^2 = r^2

and also

(x-h)^2 + (y-k)^2 =r^2

The thing is, I'm not really sure which to use, or when. Can someone help me out and explain this to me?
2. First equation is a circle centered at the origin.

Second equation is a circle centered at (h, k).

If the center of the circle matters, use the 2nd equation, if you don't care about where the circle is (e.g. you just want to calculate the area of a circle of radius r), use the 1st.
3. (Original post by Gabzinc)
I've heard two different formulae,

x^2 + y^2 = r^2

and also

(x-h)^2 + (y-k)^2 =r^2

The thing is, I'm not really sure which to use, or when. Can someone help me out and explain this to me?
(x-h)^2 + (y-k)^2 =r^2 is the general equation of a circle with centre (h,k) radius =r ,

x^2 + y^2 = r^2 h=0, is an equation of a a particular circle where k=0, h=0 , so the centre of that circle is (0,0).
4. (Original post by NotNotBatman)
(x-h)^2 + (y-k)^2 =r^2 is the general equation of a circle with centre (h,k) radius =r ,

x^2 + y^2 = r^2 h=0, is an equation of a a particular circle where k=0, h=0 , so the centre of that circle is (0,0).
Ok thanks. So should I just ignore the latter and stick with the first one as it works for all circles?
5. (Original post by Gabzinc)
Ok thanks. So should I just ignore the latter and stick with the first one as it works for all circles?
Yes.

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