Question: computing partial derivative against elements in a vector variable

I have a simple function as written below and I want to compute the partial derivative of it against an element u[k] in a vector variable u. I intentionally left k unspecified because it is not a set number and I don't really want to put it there to generate a bunch of specific output. But when I run it in Maple 12, it gives me the wrong result (sum((x-a[k])*(y-b[k]),k) instead of the correct result (x-a[k])*(y-b[k]).


Could someone shed some light on it? Thanks.

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