A function is called analytic if it can be expressed as an infinite power series around some point a. Taylor's theorem gives us a formula for the coefficients of the power series expansion of an analytic function:
Taylor's Theorem
If f(x)=∞∑n=0cn(x−a)n, then cn=f(n)(a)n!.
This formula works both ways: if we know the n-th derivative, we can figure out cn; if we know cn, we can figure out the n-th derivative.