# how can I get the right derivatives in this Sum?

I want to get the derivatives of $q_i$ of the following function Q in Mathematica: $$Q=\sum _{i=1}^N q_i$$, Using the command of $$\frac{\partial Q}{\partial q_i}$$, the software give me an answer of $$N$$, but what I really want is $$1$$, can any genius help me with it? I am working on my diploma articles, thx a lot.

The Mathematica screen is this:

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• Do not use N as variable in Mathematica, it is special symbol. – Nasser Jul 21 '16 at 6:54

When you sum it over there is no free variable. If you use the same index, Mathematica is converting the derivative of a sum to a sum of derivatives which give you n. Choose a number j such that 1<j<n and you will see your answer. For example
D[Sum[q[i], {i, 10}], q[3]]