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Differentials 

The basic idea of differentials is based in notation. By now you know that, given a function \(y=f(x)\), we write the derivative with respect to x as \(\displaystyle{ \frac{dy}{dx} = f'(x) }\). Remember that the notation \(\displaystyle{ \frac{dy}{dx} }\) does NOT mean dy divided by dx. The notation is actually shorthand for \(\displaystyle{ \frac{d}{dx}[y] }\).
HOWEVER, that said, we can actually write this a bit differently as \(\displaystyle{ \frac{dy}{dx} = f'(x) \to dy = f'(x)~dx }\). This notation gives the impression that we are dividing dy by dx but it takes a lot of math to make this transition. So please don't take this for granted.
This new form of the derivative is called the differential form. For examples see Paul's Online Notes Differentials page.
Note  There are other details about differentials that we do not yet cover here. Paul's Online Notes Differentials page has some explanation and examples. Also, here is a playlist with some videos that may help you.