Answers

2016-07-16T19:46:28+08:00
Sets can themselves be elements. 

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2016-07-16T19:47:58+08:00
It means prime. All numbers not used or included in another set.

For example:

[Universal Set] U= { 1,2,3,4,5,6,7,8,9,10 }

a={ all even numbers from 1 to 10 } -->  a={ 2,4,6,8,10 } 

a'= { 1,3,5,7,9 }
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