[tomek_ycomb]

UHHHHHhhhh, it's because the last A*b is the only one that becomes a linear constant. For other polynomials, your derivative is a polynomial still, just different one.

These are mathematical derivatives, I think of them as the slope of the thing it's derived of, aka the change in the thing that it's a derivation of.

I think I don't have a sophisticated mathematical understanding, but my basic mechanic understanding makes it feel simpler than your question is acting.