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Student Support Forum: 'Diff does not expand across summation?' topicStudent Support Forum > General > "Diff does not expand across summation?"

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Taavo Raykoff
10/16/01 05:03am

First,
In[1]:= D[(Y[i] x^2), x] yields;
Out[1]= 2 x Y[i]

Which is quite as expected. But,

In[3]:= D[Sum[(Y[i] x^2), {i,1,n}], x] yields;

2
Out[3]= Sum[D[Y[i] x , x], {i, 1, n}]

-- Why does the differentiation not resolve within the summation? Y[i] is a constant wrt x.

Thanks!!
Taavo.

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