If you want to use the managed API (which means that you are building a NETLOAD-able DLL, not an .EXE), this is probably the easiest way to do what you need, without having to do the geometry calculations:
Using Autodesk.AutoCAD.Geometry;
Point3d startPoint = //... start point of the arc
Point3d pointOnArc = //... any point on the arc segment
Point3d endPoint = //... end point of the arc
CircularArc3d arc = new CircularArc3d( startPoint, pointOnArc, endPoint );
From there, you can query the properties of the CircularArc3d for its center, radius, and start/end angles, and use them to create an Arc entity.
If you've never used the managed ObjectARX API before, then be forewarned that it is not easy to learn (it can take years to learn it to the extent needed to develop real applications).
You can ease your way into the managed API gradually by using it in conjunction with ActiveX (your problem is a good example of that, because you can create the ARC through ActiveX, but use the managed API to do the needed calculations).
And contrary to what Laurie says, we generally don't link programming languages to APIs here. This newsgroup focuses mainly on using the managed ObjectARX API without regard for langauge. For ActiveX, your best bet is the VBA newsgroup because syntax differences aside, the API is the same regardless of what langauge you use.
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wrote in message news:5838744@discussion.autodesk.com...
Thank you all for the tips, Indeed those were very helpful :))