So let's say we want to take the indefinite integral of two x plus one times the square root of x squared plus x dx. Does u-substitution apply here? And if it does, how would you define the u? Pause the video and try to think about that. Well, we just have to remind ourselves that u-substitution is really trying to undo the chain rule. If we remind ourselves what the chain rule tells us, it says, look, if we have a composite function, let's say f of g of x, f of g of x, and we take the derivative of that with respect to x, that that is going to be equal to the derivative of the outside function with respect to the inside function, so f prime of g of x times the derivative of the inside function, times the derivative of the inside function. And so u-substitution is all about, well, do we see a pattern like that inside the integral?