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Homework Help: Points where tangent line touches 2 circles

  1. Nov 13, 2008 #1
    1. The problem statement, all variables and given/known data
    On the circles y^2+x^2=1 and y^2+(x-3)^2=4
    There is a line with positive slope that is tangent to both circles. Determine the points at which this tangent touches each circle.

    2. Relevant equations
    the derivative of the first circle i found:

    the derivative of the second cirlce I found:

    and also

    3. The attempt at a solution

    so I have these equations for slopes:



    Now where do I go from here, can someone get me started with the rearranging or whatever?
  2. jcsd
  3. Nov 13, 2008 #2


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    What I think you should do is put the tangent as y=mx+c, then solve this with y^2+x^2=1, and when you simplify it into the form ax^2+bx+c=0, b^2-4ac=0 since it is a tangent. Then do the same with y^2+(x-3)^2=4 and you will get two equations in m and c.
  4. Nov 13, 2008 #3


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    I think the main problem is that you haven't written down any equations yet. Those are expressions, not equations. Put in some equal signs. Do you want to say that all of those are equal to the unknown slope m?
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