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pre-calculus

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Determine the center and the radius of the following circle:

x^2+y^2+8x-2y=8

so I figured I'd use completing the square after putting it in standard form:

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

I'm not sure where to go from here.

  • pre-calculus -

    but you didn't "complete" the square

    (x^2+8x)+(y^2-2y)=8
    (x^2+8x + 16)+(y^2-2y + 1)= 8 + 16 + 1
    (x+4)^2 + (y-1)^2 = 25

    Now we have it in standard form.
    Can you take it from there ?

  • pre-calculus -

    Ok. Do I take the square root of the whole thing now?

    getting x+4-y-1+-sqrt25
    x-y=3+-sqrt25

    I've never done this before, I'm not sure if this is even right and if it is, I definitely don't know where to go from here

  • pre-calculus -

    no,
    the standard equation of a circle with centre (h,k) and radius r is
    (x-h)^2 + (y-k)^2 = r^2

    so we ended up with
    (x+4)^2 + (y-1)^2 = 25 or
    (x+4)^2 + (y-1)^2 = 5^2

    then the centre is (-4,1) and radius is 5

    Here is an interesting page for your topic
    http://www.analyzemath.com/CircleEq/Tutorials.html

  • pre-calculus -

    Thanks so much for your explanation. I was able to figure it out and now got your answer.

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