Over here he calls it a definition.

]]>is the total differential of the function f(x,y). Why?

]]>Check here about half way down.

]]>That I do not. I will see if I can find anything.

]]>I know about that one. It is useful for implicit functions.

]]>where f is a function of x and y and the deltas denote partial derivatives.

But why is this the case?

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