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Homework Help: I have a Math Question about Circles

  1. Jun 10, 2009 #1
    Here it is:
    A circle with center (–3,4) is tangent to the x-axis in the standard (x,y) coordinate plane. What is the radius of this circle?

    I have no idea where to start. I know there is some rule about tangency and circles, but I am unsure. Thanks!
  2. jcsd
  3. Jun 10, 2009 #2


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    If it's tangent to the x-axis, then it touches the x-axis at precisely one point. Try drawing a circle and you should be able to figure out which point it touches the x-axis at
  4. Jun 10, 2009 #3
    You really dont even need to draw a circle; you should be able to intuit the answer. If the mp is (-3,4) and it only touches 1 point on the x-axis, what is the radius. Isnt the (sort of in this case) definition of radius a ray as a line starting from a (mid)point and going infinitiely in one direction, in this case until it touches the axis? Then how far is that?
  5. Jun 10, 2009 #4


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    A radius is always perpendicular to a tangent line. Here, the x-axis is a tangent line. Therefore, a radius must be perpendicular to the x-axis. If the center is at (-3, 4), what is the other end of that radius? How long is it?
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