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Question about evaluation with Sum


Hello,


I have a big problem of evaluation with the function Sum.

With the very simple following example, 

   p=k

   Sum[p,{k, 1, n}]
    
     p k

   Sum[Evaluate[p], {k, 1, n}]
   
     1
     - n (1+n)
     2

   Sum[p,{k, 1, 5}]

     15

how can I explain my students that 
   
   in the first case p is left unevaluate and it needs Evaluate,

   in the last case p is evaluate correctly  ?

I do not understand how the "Attributes" of Sum can explain this.


Thank You in advance


Jack Michel CORNIL

Versailles FRANCE


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