The Modify tools - Introduction to AutoCAD 2011: 2D and 3D Design - page 120

Select objects or <select all>: pick 1 found
Select objects: right-click
Select object to extend or shift-select to trim
or[Fence/Crossing/Project/Edge/Undo]: pick

Repeat for each object to be extended. Then:
Select object to extend or shift-select to trim or
[Fence/Crossing/Project/Edge/Undo]: right-click
Command:

The results are shown in Fig. 5.40 .

Fig. 5.39 The Extend tool icon from the Home/Modify panel

objects to
extend

boundary edge

Results

objects to
extend

Fig. 5.40 Examples - Extend