Modify > Break (select from Main Menu)
Break (click icon on Modify Objects Bar)
Breaks a primitive entity at one or two locations.
The following entities may be broken: Polyline, Line, Circle, Arc
Usage:
1.Enter the Break command (from the menu or toolbar).
2.Indicate the first point where the object should be broken.
3.Optionally indicate the second point where the object should be broken. Press Enter or right-click to break the object at a single point.