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    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?
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    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.
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    (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?
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    (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.
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    (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 ...
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    (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.
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    (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.
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    What the graph looks like. Not a circle.
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    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)
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    yep done it :d i made a sign mistake
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    Schoolboy error :P
 
 
 
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Updated: March 31, 2011
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