Location: Modify>Fillet>No Trim
Use the Fillet and No Trim function to create a fillet arc without trimming.
Using the Function:
Select No Trim from the Modify>Fillet submenu.
Enter a value for the radius of the arc fillet.
Select the first fillet entity. (You can click anywhere on the entity; you do not have to select any specific location relative to the arc radius.)
Select the second curve on the portion of the curve that you want to remain.
Above > An example of Fillet and No Trim