Tangent Line to Two Circles

[Markel]

One of our distributors called me about wanting this. He wants to measure two circles and construct the exteriour tangent on one side of them. (I work with precision measuring products.) I've tried searching for an easy formula to calculate this (or the two coordinates of the tangent points), but haven't had much luck.

So, if I have Circle A and Circle B and know the center coordinates and radii of both, how can I find the coordinates of the contact points of the exterior tangent?

[BrewMaster]

i swear we did something like this in a math class i took in college, but i seriously can't remember how to do it. good luck Markel.

[InfiniteNothing]

Exterior tangents or the interior tangents (there's 4 of them... 2 that cross in the center and two that don't) Uhhhh Oxo and Oo> if that makes sense


... just saw you answered that question. Hold for solution

[hapoo]

I could write a program to do it :)
but it would be a little tuffer to find the formula for it.
kinda weird how i can solve a problem that i don't know the formula for :)

[InfiniteNothing]

CC is center of the big circle, Cc is center of the little cirle. a is the distance between the circles. I have little circle to the left of big circle. R is radius of the big circle, r is the radius of the small one.

http://home.earthlink.net/~chemeng05/math/c.png
http://home.earthlink.net/~chemeng05/math/ymx.png
http://home.earthlink.net/~chemeng05/math/center2.png
http://home.earthlink.net/~chemeng05/math/center1.png

Edit: Corrected and tested. I'd show you my work but I don't have a scanner. FYI C is the distance between the tangent points

[hapoo]

So infinite, hows this working out for you?? :P:P
http://home.earthlink.net/~chemeng05/BestSeller.jpg

[InfiniteNothing]

So infinite, hows this working out for you?? :P:P
http://home.earthlink.net/~chemeng05/BestSeller.jpg

Don't you find it the least bit funny that it's sales rank #1