I'm reading through my textbook, and I am not quite following the derivation of Simpson's rule. The one in my textbook is almost identical to the derivation on this website:

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I just can't figure out why the term "(Bx^2)/2" just dissapears! In my textbook it's there on one line and the next line it's just gone and that's all that changed between the two lines. Could someone please explain to me why this happens? Thank you!