" (dy/dx) = (dy/du)(du/dx) Caution: Notice that if (dy/du) and (du/dx) were quotients rather than expressions for derivatives, we could cancel the du`s and make (5) into an identity. But we stree that du has not been defined as an entity, and consequently it is not legitimate to cancel the du`s. Nonetheless, the resemblance between (5) and an algebraic identity makes it easy to remember. " so, he`s saying that I can`t cancel out the du`s, because they are not quotients, but expressions for derivative.. but I think that they can be canceled out!? is that true? and yes they are expressions for derivative, but also they are quotients.. and I understand that we need them, and it would be meaningless to cancel them.. its just that I think he`s giving me wrong information when he said that I "can`t cancel them out" because "they are expressions for derivatives" Am I wrong?