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Some questions about vector


input:

Clear[f];
f[v] = Sum[(i*v[i]), {i, 1, N}]
D[f[v], v[i]]

the output is

\!\(1\/2\ N\ \((1 + N)\)\)

However what I expect is i

What should I do in order to get the the answer I expected?

Thanks a lot.

-- 
Best,
Qi Zhang


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