title: How to show summation function in Mathematica Instead of expand the whole series?
body: <p>I am trying to compute the derivative for my homework assignment, and the homework is using a Summation series where i have shown in the first line of my code.</p>
  
  <p>Mathematica expanded all terms for me in this series. I am wondering if there is a way i can make it return the result using an Epsilon sign.</p>
  
  <pre><code>f[x_]=Sum[k*Exp[-Subscript[a,k]*x^3],{k,1,7}]
  
  E^(-x^3 Subscript[a, 1]) + 2 E^(-x^3 Subscript[a, 2]) + 
   3 E^(-x^3 Subscript[a, 3]) + 4 E^(-x^3 Subscript[a, 4]) + 
   5 E^(-x^3 Subscript[a, 5]) + 6 E^(-x^3 Subscript[a, 6]) + 
   7 E^(-x^3 Subscript[a, 7])
  
  f'[x]
  
  -3 E^(-x^3 Subscript[a, 1]) x^2 Subscript[a, 1] - 
   6 E^(-x^3 Subscript[a, 2]) x^2 Subscript[a, 2] - 
   9 E^(-x^3 Subscript[a, 3]) x^2 Subscript[a, 3] - 
   12 E^(-x^3 Subscript[a, 4]) x^2 Subscript[a, 4] - 
   15 E^(-x^3 Subscript[a, 5]) x^2 Subscript[a, 5] - 
   18 E^(-x^3 Subscript[a, 6]) x^2 Subscript[a, 6] - 
   21 E^(-x^3 Subscript[a, 7]) x^2 Subscript[a, 7]
  </code></pre>
  
  <p>Many thanks!</p>