Hi Felix, maybe I'm a little bit lost with what you're discussing here, but like Burr says above, MoI does have a tan/tan snap that you can use to construct these kinds of lines without needing to do any of that additional extra work.

> here is a simple 2d drawing where I was trying to create the blue lines. For what I would call the "external"
> tangent lines, the solution is to create a circle inside the larger circle with a radius of R2-R1 then draw a
> line from the center of the small circle tangent to this circle we just created and translate this line to the
> edge of small circle (followind the perdendicular radius).

You don't need to do any of this additional circle construction and line translation - just draw in a single line and it will give you a tan/tan snap point that you can grab on to. It's a special snap that not only snaps the endpoint of the line but also moves the base point of the line as well. It also works on bendy curves as well, not just circles.

In order to use it make sure "Object Snap" is enabled in the bottom toolbar, and also make sure you have not disabled "tan/tan" snap type in the object snaps list, and then you just draw in a line nearby where the tan/tan line is and move your mouse until you find the snap point that is created, it is usually just a short distance away from the single end-point tangent line.

See here for a previous example:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2788.4

EDIT: and I was going to do a video showing it on your same file too but it looks like Burr has done that already, thanks Burr!

So none of that extra work for constructing circles and moving lines is needed, just draw a single line in fairly close to where the tan/tan line is at and there will be a snap point created that you can move your mouse to which will then give you the tan/tan line, it works on curves as well.

- Michael