Informal Derivation of Taylor Series

We have a function
and we want to approximate it using an
th-order *polynomial*:

where , the approximation error, is called the

Our problem is to find fixed constants so as to obtain the best approximation possible. Let's proceed optimistically as though the approximation will be perfect, and assume for all ( ), given the right values of . Then at we must have

That's one constant down and to go! Now let's look at the first derivative of with respect to , again assuming that :

Evaluating this at gives

In the same way, we find

where denotes the th derivative of with respect to , evaluated at . Solving the above relations for the desired constants yields

Thus, defining (as it always is), we have derived the following polynomial approximation:

This is the th-order Taylor series expansion of about the point . Its derivation was quite simple. The hard part is showing that the approximation error (remainder term ) is small over a wide interval of values. Another ``math job'' is to determine the conditions under which the approximation error approaches zero for all as the order goes to infinity. The main point to note here is that the Taylor series itself is simple to derive.

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