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Math Help - Equation of circle

  1. #1
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    Equation of circle

    Find the equation of the circle which passes through (-1, 4)and with center at (4,-8)

    I know this is an easy question but I just cant find the formula for how to find the equation. Any help appreciated.
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  2. #2
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    I think you'd find the distance from the center to that point. That's the radius.
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  3. #3
    Senior Member topher0805's Avatar
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    The standard form for the equation of a circle is:

    (x - h)^2 + (y - k)^2 = r^2

    Where the point (h,k) is the center of the circle.

    We are given that the center is found at (4,-8) so we have our equation so far:

    (x - 4)^2 + (y + 8)^2 = r^2

    Now we just need to find the radius. We have been given one point on the circle, so to find the radius we simply need to find the distance from the center of the circle to that point. Recall the distance formula:

    d = \sqrt {(x_2-x_1)^2+(y_2-y_1)^2}

    Plug your values in to find the radius, then simply enter that into the previous equation to get your final answer.
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  4. #4
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    Ah. The answer is (x-4)^2 + (y+8)^2 = 169 from what you said about the distance from those 2 points.
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  5. #5
    Senior Member topher0805's Avatar
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    That is correct.
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