Taylor Series Approximation to Cosine
If a function has a Taylor series that is convergent to the function, it is customary to expect that partial sums with more terms provide a better approximation than those with fewer terms. As the example of y = cos(x) shows, this statement must be qualified.
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For x large in absolute value, higher degree polynomials provide worse approximation than lower degree polynomials. For such x, the best approximation is given by the constant term y = 1.