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    a circle passes through (0,2) and (0,8) and its centre lies on the line x=4.

    what is the equation of the circle?

    okay so I drew a sketch and appreciated that the centre of the circle must be (4,y)

    but how do I find y? and how do I find the radius? thanks
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    (Original post by madfish)
    a circle passes through (0,2) and (0,8) and its centre lies on the line x=4.

    what is the equation of the circle?

    okay so I drew a sketch and appreciated that the centre of the circle must be (4,y)

    but how do I find y? and how do I find the radius? thanks
    The distance from the centre of a circle to any point on its circumference is always the same - you can use this fact to find y.
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    (Original post by Mark13)
    The distance from the centre of a circle to any point on its circumference is always the same - you can use this fact to find y.
    so, y would be 4 too?
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    (Original post by madfish)
    so, y would be 4 too?
    No.

    The distance from the centre to any point on the circle is always the same (the radius). Meaning:

    Distance from (4,y) to (0,2) = Distance from (4,y) to (0,8)
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    (Original post by justinawe)
    No.

    The distance from the centre to any point on the circle is always the same (the radius). Meaning:

    Distance from (4,y) to (0,2) = Distance from (4,y) to (0,8)
    how do I find y from this?
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    (Original post by madfish)
    how do I find y from this?
    Don't you know the distance formula?
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    The equation of a circle is (x-h)^2 + (y-k)^2=r^2 where (h,k) are the coordinates of the centre of the circle, and x and y are the coordinates of any point on the circle, and r is the radius. Using the information you have, you will get two equations. Solve simultaneously to find k (the y coordinate of the centre of the circle) and then find r

    *Yeah this is slightly more complicated, but it will work for any other question too :P
 
 
 
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