In solution for part iv, when we differentiate the RHS of (iii) wrt x_i, what happened to the summands? Is this how we differentiate a summation? Thanks.
Q&A Bank 5.10 When you write out the summation in full, you get terms in x(1), x(2), ..., x(n). When you differentiate wrt x(1), the only term giving a non-zero result is the first one, so we don't need a summation in the first term once we've differentiated.
