In Response To 'Re: Help - Derivative of Sums'
I think I now understand what is happening here.
The Sum of D[Subscript[xm,i],Subscript[xm,i]] is expanded and cancels the k.
D[1/k Sum(i = 1,k) Subscript[xm,i],] ==
1/k Sum(i = 1,k) D[Subscript[xm,i],] ==
(1/k) (1+1+1..1) ==
(1/k) (k*1) ==
The lower-case L from the xw Sum is still in the numerator.
The attached notebook has an expanded version.
Attachment: stu23594problemC.nb, URL: ,