Fillet and No Trim
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Use the Fillet and No Trim function to create a fillet arc without trimming.
Using the Function:
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Select No Trim from the Modify>Fillet submenu.
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Enter a value for the radius of the arc fillet.
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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.)
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Select the second curve on the portion of the curve that you want to remain.
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Above > An example of Fillet and No Trim