Maxima: differentiate the amount at a specific index position

Question

In Maxima 12.04.0 I have a sum

mysum : sum(u[i]^2, i, 1, N); 

now I differentiate it

 diff(mysum, u[i]); 

I now set a specific index i = A to distinguish it from

 at(%, i=A); 

Unfortunately, the maximum does not replace u [i] in total in this way.

How can I bring highs to a result like

 2*u[A] 

Answer 1

After you split, press 2 in the sum, select the i-th term and then replace i = A:

 (%i1) mysum : sum(u[i]^2, i, 1, N); diff(mysum, u[i]); intosum(%); part(%, 1); %, i=A;