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Homework Help: Circle Geometry

  1. May 24, 2008 #1
    1. The problem statement, all variables and given/known data

    The circle C, with the centre at the point A, has equation x² + y² - 10x + 9 = 0


    a) the co-ordinates of A,

    b) the radius of C

    2. Relevant equations

    (x-a)² + (y-b)² = r²

    3. The attempt at a solution

    ... not sure what to do really. any suggestions

    thanks :)
  2. jcsd
  3. May 24, 2008 #2
    Once you do one of these, you'll know how to do them all. Try completing the square for both variables.
  4. May 24, 2008 #3
    (x²-5) -16

    so does this mean (5,0) is the co-ordinate.
    is the radius sqrt(16-9) = sqrt(7)


  5. May 24, 2008 #4
    You have a x2 and a y2. How would you get it into the form (x-a)2 and (y-b)2?
  6. May 25, 2008 #5
    sorry im not sure :( did i complete the square correctly?
  7. May 25, 2008 #6


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    Homework Helper

    It's sort of only a bit correct. The equation of a circle is [itex](x-x_0)^2 + (y-y_0)^2 = r^2[/itex], where [itex]x_0[/itex] and [itex]y_0[/itex] are the coordinates of the centre and r is the radius.

    Complete the square for x^2 - 10x + 9 first, then substitute the resulting expression back into the question, and move the constant to the RHS. You'll get the equation of circle.
  8. May 25, 2008 #7
    Thanks!!! :)
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