Common tangent

  1. 1. The problem statement, all variables and given/known data

    Is there any direct formula for calculating the direct common tangent of two circles without having to go all the trouble of using y-y1=m(x1-x2) to derive it for two seperate tangents t1 and t2. If there is could anyone explain to me how it is derived???

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. ehild

    ehild 12,950
    Homework Helper
    Gold Member
    2014 Award

    How would you draw those common tangents?

    The problem can be reduced to the problem of drawing tangent from a point. Shrink (or blow up) the radius of the bigger circle by the radius of the smaller one. The tangent to the new circle from the centre of the smaller circle e is parallel to tangent line t.
    It is easy to get angle theta from the distance of the centres (d) and the difference of the radii. Write up the equation of the green lines. They intersect the tangent points on the circles.


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