If you want to draw accurate arcs/circles, tangents, intersections etc etc then you might try my 'True Tangents' - the tools in the script do various tasks, including giving you the true intersection cpoint[s] between a selected arc/circle and a line which you can then use for a replacement arc/circle and edge that will have proper intersections with the line...
See here http://forums.sketchucation.com/viewtopic.php?p=160780#p160780