Differentials in Linear Approximation - Expii

You can phrase linear approximation (of a function f(x) at a point x=a) in terms of the differentials dx and df (or maybe dy instead of df, if you are working with the graph representation y=f(x)). The point is that the graph y=f(x) is approximately the line of slope f'(a) passing through point (a,f(a)).