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Fie multimea A={x|13

Răspuns :

13<x<=28
=> x∈(13, 28]
x este nr natural:
    => x∈{14, 15, 16, 17, 18, 19, 20,  21, 22, 23, 24, 25, 26, 27, 28} 
A={14,15,16,17,18,19,20,21,22,23,24,25,26,27,28} 
Foarte simplu!