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Math Help - [SOLVED] Equation of Circle

  1. #1
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    Smile [SOLVED] Equation of Circle

    Im having trouble answering questions like this, any help would be much appreciated.

    Find the centre and radius of the circle with equation

    x^2+y^2-6x-2y-6=0
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    Bar0n janvdl's Avatar
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    Quote Originally Posted by edwards2779 View Post
    Im having trouble answering questions like this, any help would be much appreciated.

    Find the centre and radius of the circle with equation

    x^2+y^2-6x-2y-6=0
    x^2 - 6x + 9 + y^2 - 2y + 1 = 0 + 9 + 1 + 6

    (x - 3)^2 + (y - 1)^2 = 16

    I'll also give it to you in LaTeX. Just notice the constants I've marked in red.


    x^2 - 6x + 9 + y^2 - 2y + 1= 0 + 9 + 1 + 6

    (x - 3)^2 + (y - 1)^2 = 16
    Last edited by janvdl; February 24th 2009 at 08:54 AM. Reason: Bummer. Forgot the 6.
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  3. #3
    Like a stone-audioslave ADARSH's Avatar
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    For
    x^2 +y^2 + 2hx +2fy +c =0

    Center is (-h,-f)

    And Radius is

     <br />
\sqrt{h^2 +f^2 -c}<br />


    EDIT: Too late Jan watch "6"
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  4. #4
    MHF Contributor Reckoner's Avatar
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    Smile

    Quote Originally Posted by edwards2779 View Post
    Im having trouble answering questions like this, any help would be much appreciated.

    Find the centre and radius of the circle with equation

    x^2+y^2-6x-2y-6=0
    If you are not familiar with the technique of completing the square, then you should review it.

    In this example, we have

    x^2+y^2-6x-2y-6=0

    \Rightarrow(x^2-6x+9)+(y^2-2y+1)=16

    \Rightarrow(x-3)^2+(y-1)^2=4^2\text.

    Now can you see the center and radius?
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  5. #5
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    oh i see now, so you basically just factorising. thanks for the help
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