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Math help! Circles and slope

  1. Feb 10, 2008 #1
    1.) How do you answer this?
    Find the equation of the line that has a slope undefined and passes through the points (3, -4).

    I got y=-4
    2.) Write the standard and general equations of the circle with radius 4 and center (1,2).
    I got STANDARD: (x-1)^2 + (y-2)^2=16
    I don't understand the whole GENERAL FORM!
    3.) Then I have this: Give the center, radius, and all intercepts of the circle: x^2+y^2+4x-2y-20=0.
    I am stuck on this one completely.

    Thank you for all help!!!!!!!!!!!!!!!!
  2. jcsd
  3. Feb 10, 2008 #2
    1. What is the point-slope equation?

    2. Correct.

    3. Complete the square, put it in general form, which C(h,k) is the center.

    General form just basically tells us where the center is, and the length of the radius. If I were to ask you the difference between [tex]y=x-1[/tex] & [tex]y=x+1[/tex]. You would say the first is shifted a unit down, while the 2nd is shifted up a unit. Same thing for the circle. For example: [tex](x-1)^2+(y-2)^2=4[/tex], shifted to the right 1 & up 2 with radius 2.
    Last edited: Feb 10, 2008
  4. Feb 10, 2008 #3


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    #1: You want a slope undefined. Your answer is a slope of zero. They are not the same.

    #2: You obtained the correct equation in standard form, but you want the final answer in GENERAL form. Perform the steps (meaning, multiply!)
  5. Feb 10, 2008 #4
    Crap! Sorry, I'm a dummy. General form is what they have in question 3. Standard form is C(h,k) form, hehe.
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