To Create an Approximate Surface Extension
Use this procedure to create an 
Along Original Surface extension using the Approximate option.
Along Original Surface extension using the Approximate option.1. Perform one of the following actions:
◦ Select the boundary edge chain of the surface that you want to extend, and click > 
Extend.
Extend.◦ Click > Extend, and then select the boundary edge chain of the surface that you want to extend.
Extend, and then select the boundary edge chain of the surface that you want to extend.The Extend tab opens with the Along Original Surface type selected.
Along Original Surface type selected.2. Click the Options tab, and select Approximate in the Method box.
3. For each extension side, select Along to create the side along the side edge, or select Normal To to create the extension normal to the boundary edge.
4. To extend the surface, you can:
◦ In the graphics window, use the drag handle to manually extend the selected boundary chain to the desired distance.
◦ On the Extend tab, type a distance value in the 
Extend by box.
Extend by box. | You can click  at any time to toggle between extending or trimming the original surface. |
5. To add measurement points, click the Measurements tab. Right-click inside the tab and choose Add. Define the point's location and use the point to drag the extension in the graphics window or set the location and extension on the Measurements tab. You can add multiple points for finer control.
6. Click 
OK.
OK.