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Add or subtract the coefficient or the number before each radical,  then copy the radical for similar radicals.  Note that if you don't see a coefficient, assume there is a number "1".

The rule is, you can only subtract or add radical expression with same index  and radicand.

Ex:    4∛21  +  ∛21  =  5 ∛21   
          Note that if there is no coefficient before the radical, assume there is number 1