Assuming a helper line between the centers of the two circles, the four possible tangent lines are:
1. Left outer edge of circle 1 to the left outer edge of circle 2
2. Left outer edge of circle 1 to the right outer edge of circle 2 (a diagonal, if you will)
3. Right outer edge of circle 1 to the left outer edge of circle 2 (the other diagonal)
4. Right outer edge of circle 1 to the right outer edge of circle 2
To find these equations, you need to provide two X values and then solve for Y for each of the four lines above, to calculate the A, B & C values you need.
In order to find the X value, you need to find two normals for each of these lines -- the first normal extends from the center of circle 1 to the point on the circumference where it will be perpendicular to the line that joins the other circle. The second normal extends from the center of circle 2 to the point on it's circumference where it will perpendicular to the same join line.
Repeat for the other three lines.
The simplest case is where both circles are the same diameter, then the point where the normal and the join line meet is merely the centerpoint of the circle minus the radius (the normal would follow along an X or Y axis, making the math easy!).
Chip H.
____________________________________________________________________
Donate to Katrina relief:
If you want to get the best response to a question, please read FAQ222-2244 first