If we say u is equal to x squared plus x, then we could say du dx, the derivative of u with respect to x, is equal to two x plus one. If we treat our differentials like variables or numbers, we can multiply both sides by dx, which is a little bit of hand-wavy mathematics, but it's appropriate here. So we could say two x plus one times dx. And now what's really interesting is here is we have our u right over there, and notice we have our two x plus one times dx. In fact, it's not conventional to see an integral rewritten the way I'm about to write it, but I will. I could rewrite this integral. You should really view this as the product of three things.