Sign confusion in partial differentiation of a summation.

Hello all, thanks for taking the time to read this.

So, I am looking over a document about least squares fitting, and right in the beginning is a summation function:

Attachment 24099

This function is partially differentiated w/respect to b to get:

Attachment 24100

Which is fine. However, here’s where I am confused: the simplified function appears as:

Attachment 24101

Shouldn’t the last term be positive, as in:

Attachment 24102

or am I missing something?


Thanks for your help!