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# Equation of a circle help watch

1. So circles has never been my strong point we are told to show the equation (x+y)(x+4)=X-y)(x-2) represnts a circle and find the radius and coordinates of its center. From the equation i cna find teh radius and coordinates its just i dont no how to show it is the equation of a circle any help?
2. The general form for the equation of a circle is (x-a)^2 + (y-b)^2 = r^2, where (a,b) is the centre of the circle and r is the radius (if you draw a picture, this is just pythagoras).

So to show your equation represents a circle, you need to manipulate it into the above form. Once it's in that form, you can just read off the radius and coordinates of the centre.
3. (Original post by Daniel Freedman)
The general form for the equation of a circle is (x-a)^2 + (y-b)^2 = r^2, where (a,b) is the centre of the circle and r is the radius (if you draw a picture, this is just pythagoras).

So to show your equation represents a circle, you need to manipulate it into the above form. Once it's in that form, you can just read off the radius and coordinates of the centre.
I get x^2+2x+y^2+2y=-2 does this seem correct?
4. (Original post by hazbaz)
I get x^2+2x+y^2+2y=-2 does this seem correct?
What you have written there might be correct (I haven't done the working myself), but it's still not quite in the right form.

(the -2 on the RHS should shout at you. how can the radius squared be -2?)

You need to have (x-a)^2 + (y-b)^2 = r^2. Try completing the square on the x terms, and then the y terms.
5. (Original post by Daniel Freedman)
What you have written there might be correct (I haven't done the working myself), but it's still not quite in the right form.

(the -2 on the RHS should shout at you. how can the radius squared be -2?)

You need to have (x-a)^2 + (y-b)^2 = r^2. Try completing the square on the x terms, and then the y terms.
So youget (x+1)^2 +(y+1)^2 +2=0 ...
6. (Original post by hazbaz)
So youget (x+1)^2 +(y+1)^2 +2=0 ...
Now that I've done the working, I see that the question is wrong. It's not a circle, as there are no x^2's once you expand it out.

Have you typed it out incorrectly?

Ignoring all of this, you've completed the square incorrectly anyway.
7. (Original post by Daniel Freedman)
Now that I've done the working, I see that the question is wrong. It's not a circle, as there are no x^2's once you expand it out,

Have you typed it out incorrectly?
Nor are there any y^2s either.

8. What the graph looks like. Not a circle.
9. Its typd out correctly i think ive done it...

I did type it out incorretly it was (x+y)(x+4)=(x-y)(y-2)
10. yep done it :d i made a sign mistake
11. Schoolboy error :P

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Updated: March 31, 2011
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