nth-degree Taylor polynomial for f centered at c
$$ P_n(x)=f(c)+f'(c)(x-c)+ \frac {f''(c)} {2!}(x-c)^2+...+\frac {f^{(n)}(c)} {n!}(x-c)^n $$
if see a_n just think of $\frac {f^(n)}{n!}$
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